I don’t think it’s necessarily clear what they meant because “en masse” has been pretty thoroughly borrowed into English as a singular phrase, as have many other French words and phrases. To then literally translate that borrowed phrase back into English is just confusing because those English words are not correct in that context (while the borrowed French phrase is).

For what it’s worth I definitely did a double take and had to re-read the sentence because I was not expecting those words there.

I like being corrected. Actually, I would consider it rude if you didn't correct me. Thank you for helping me making less mistakes in the future.