The README is more informative than this thanks to this patch here, so that is not a particularly applicable argument. I would suggest the primary reasons that someone runs cmake in the src tree is either an experienced dev making a small slip-up (so the instructions don't help), or someone who hasn't even read the briefest set of instructions on how to build WebODF, in which case this tip here is not going to improve their ignorance that much.

Meh, I'm not fussed either way, so feel free to submit as-is if you are particularly attached to these.