Post-Oyster Bay damage assessment by the League
They're not worrying about the defenses on the Junction itself. Those defenses are light-hours away from the objective they propose to attack: the inner system where all the people live. Some of those defenses could be transferred to the inner-system, but the volume to be protected there is far vaster than the volume to be protected at the Junction and the Junction defenses are primarily point defenses: fixed weapon systems with limited range and zero mobility. More, they know very little (if any) of those defenses have been transferred elsewhere specifically because they have gotten post-battle reports from ships transiting the Junction. For that matter (though this is a teeny bit of a spoiler for the next book) they have eyes continually deployed under an appropriate cover right on top of the Junction and they're keeping them there.
What they are not seeing (and cannot see) are what warships and/or missile pods have survived or been redeployed to cover the inner system. They are assuming that there can't possibly be very many of either (or at least that they have to have suffered massive losses) for any attacker to have fought his way in to inflict the damage the Manties have admitted to having suffered. The Manties are not only telling them they're wrong, they are actively warning them through their ambassador to the League that any Solly attack on the system will be annihilated by their defenses; the Sollies are interpreting this as “bold talk from a one-eyed fat man” and assuming that it is (at least in large part) a bluff being used in a desperate attempt to persuade them to call off their dogs. Filaretta's operational planning includes an option for breaking off the attack if he gets there and sees evidence that the analysts are wrong (i.e., he ain't as dim as Crandal was, but then who with a working cortex could be, aside from Joseph Byng?), but the fundamental operational assumptions are: (a) the Manties are probably lying and can't really defeat him; (b) even if they can, we still have enough SDs (which we are assembling rapidly) to hit them even harder with a second wave within a period of only a few months; (c) their morale is probably already so battered that they will roll over when they realize we're truly willing to absorb the kind of casualties they're "warning" us about and keep coming (especially since they can assume we're going to get even nastier and more vengeful if they hurt us still worse before their inevitable defeat by the Invincible Solarian League Navy); (d) each attack we send in will inflict at least some damage and force them to remain on the defense with their limited remaining warships, thus eroding their combat potential, keeping their wall of battle from coming after us, and buying time for us to adapt to their technological bag of tricks (assuming, of course, that they really have any tricks which could truly threaten the Invincible Solarian League Navy).
By the time the actual attack rolls in on Manticore, Kolokoltzov and his civilian compatriots are going to be entertaining serious doubts about Rajampet's underlying assumptions, but as they see it, they're committed. They also have some side strategies of their own in play, but Kolokoltzov's fundamental position is that (a) Rajampet may be right and (b) Filaretta is probably smart enough to back down if it turns out he's facing the short end of a battle of annihilation.
Of course, he might be wrong
Something I forgot to add. One might well argue that the fact that not much seems to have been moved from the Junction's defenses to the inner system, despite those defenses' short range and lack of mobility, should indicate that the Manties aren't especially worried about their ability to defend the inner system, which --- in turn --- should suggest the damage to those inner-system defenses cannot be as heavy as the Invincible Solarian League Navy is assuming. Hence the fact that the Sollies know they haven't redeployed those defenses should tip off the ISLN that the Manties' inner-system defenses remain intact. From the perspective of the people planning this operation, however, that ain't necessarily so. The Junction is absolutely vital to the SEM, as the heart of its economic muscle and the critical linchpin in its strategic military (and political, given the separation between the home system and the Talbott Quadrant) communications. Moreover, it's where the support industry that didn't get killed in Oyster Bay is located to service Junction traffic which, after all, is extraordinarily heavy. From the planners' perspective (and let us not forget that some/many of them have both a penchant for and a habit of indulging in wishful thinking), then, the Manties have to defend the Junction as well as the inner system. Given that basic premise, it isn't unreasonable to assume that they would leave the fixed defenses to do that while massing every bit of their remaining (battered, limited, decimated, and Hopelessly Outclassed) mobile forces to cover the inner system. IOW, the fact that the Junction defenses remain strong says nothing about the state of the inner system defenses the ISLN is actually planning to penetrate.