I thought this comment was making the opposite point, that it was impressive that Messenger was now only 360K LoC.

I honestly think it is. Compliers, DBMS, a library such as NumPy— they're all complex, but Messenger, even considering the features removed, is a very full-featured application. We're not talking IRC here. It has activity status, payments, stories, photo filters, audio messages, location sharing, video-calling, etc. It's actually pretty impressive that that fits in 360K LoC IMO.