I don't want to discuss the ins and outs of mix-n-match setups as it is pretty pointless to argue. And there will always be someone who took Nuovo Record RD, paired it to SRAM shifters, and a Suntour cassette and it shifts like glass.
But I do want to leave a comment on the Shimano rep's statement that road shifters are not compatible and won't work with mountain bike rear derailleurs. Technically this is true from Shimano's documentation. I have Shimano Product Compatibility Product manuals back to 2004. I cannot recall ever seeing any Shimano chart that mixes specifically identified road and mountain components; an exception might "technically" be GRX, but I haven't really looked at those docs.
Based on this, the Shimano rep cannot tell a customer to mix components that are not supported by Shimano Tech Docs. I imagine part of it is to sell current products, but I think the bigger part is product liability. Shimano has always been conservative in a lot of their specs/documentation and it is for good reason.
John