"Oil tanned" leather is chrome tanned leather (unless otherwise stated, I suppose). Chromexcel is combination tanned (veg/chrome) so has some properties of both.
based on that, I would say this fellow is wrong.
A lot of oil tanned leathers have a pull-up effect like Chromexcel. So they may be similar in that respect. But Chromexecl is a very specific thing from a particular manufacturer. "Oil tanned" could be anything so i guess it depends on who made it, who it's made etc.
I guess my question is what is this other oil tanned leather your supplier is selling?