In ruminants, a lot of this happens in the stomach, entirely different part of the gut. Maybe there's a reason things developed that way?

I would bet that comparable amounts of cellulose from celery would be harmless. Freeze dried vs. fresh celery might be an interesting study.

Problem is, once it's not pure cellulose, you don't know if it's that cellulose in celery is harmless (if that's the case) or if something else in the celery negates the effects.