It WAS known that Momenta would have to reimburse Sandoz for development costs upon approval of m-enox. (In fact this had been discussed on this board for weeks in advance of the approval announcement)
The specific formula for reimbursement (capped at 50% of revenue per quarter) was not known. MNTA coming out and explicitly noting that they expect to fully reimburse Sandoz in 3 quarters is also a new tidbit of information.
In light of the above, your assertion that the market sold MNTA off because of this news is without merit.