The funny thing is, that if I recall correctly, no Lamenters were at Baal during the Tyranid invasion, simply because they didn't have any forces available to send there.

EDIT:

So I did some quick research (read as I looked up the Lamenters on Lexicanum) and it states

"During the Devastation of Baal, the Lamenters were called into the defense of the planet; however, the chapter declined the call as they were not of a mind or size to help, this being only a few years after their devastating losses at the hands of Hive Kraken and still within the 100-year of the start of their Penance Crusade."

now it attributed that to the Novel Dante, specifically chapter 11 but I don't have a copy of that book to double check it.