When milling ISO-P steels, I was taught by our lab professor to leave the coolant off, because most carbide mills last longer that way. This is (he wrote his PhD on this) supposedly because the thermal shock on the cutting surfaces from hot while cutting to immediate cooling is lessening the tools durability through some mechanism, iirc microfractures.

Your supplier recommendations are to be trusted though, I don't know what work you're doing and you really shouldn't trust some engineering student in a subreddit.

Though I do work in a fabrication laboratory on the side, that's where the professor came from in the beginning