I think the Viking dealer must have had a stock of polyester thread to sell. I don't know of *any* machine that won't sew with cotton thread!
It is true, however, that cotton thread will produce more lint; polyester is almost lint-free. The dealer may have been thinking that sewists do not know how to clean lint out of their machines with regularity. I know I have bought garage sale sewing machines with hard-packed lint underneath the feed dogs; no wonder the poor dears didn't sew well!!!
As long as you understand how to do routine cleaning of your machine, there's no reason to avoid cotton thread. I like Aurifil because it produces very little lint compared to other cotton threads I have tried (including Mettler silk-finish cotton).