My immediate reaction is to wonder whether he/she is privy to some information I'm not which would make X, Y, and Z a priority, whether he/she is overlooking or ignorant of some other factor which would make them not worth doing, or some combination of the two. As such, I'd likely try to strike up a conversation to see if we could reach a mutual understanding. I'd certainly hate to be working on the wrong thing because I was the one missing something.

I suppose if it happened regularly enough, and it was clear the founder usually spoke from a position of ignorance, I'd eventually disengage and start to dismiss them out of hand, but I can't say I'd ever take it personally.

While the founder could just as easily have said "I think we should do X, Y, and Z" instead, I'm more than a little concerned at how easily someone would shut down over the original statement, which strikes me as mostly harmless.

I don't know your old founder and can't speak for them, but generally, that kind of phrasing is just idiomatic, and not meant to be parsed literally.

It's just a colloquial way of saying "I've earnestly thought about X, Y, and Z and think it's the most effective way path forward" or "my intuition is telling me X, Y, Z is the best way to go"

It's a way of expressing that they have a strong sense of how to proceed and the "passive-aggressive jab" that you hear is just an artifact of the idiom not being familiar or natural to you.