Yes, and then you show the visa. Sorry, no idea what you want to argue about.
The only visa where an onward ticket is required is a single entry tourist visa. Which at the moment no one is handing out, as most countries fall into visa exempt and you get a 2month free stamp.
So, if "what ever airline you are using" is asking for an onward ticket, when you actually have "a real visa", they are in mistake. And it should be simple to point that out at the airport.
A DTV visa does allow it and is a proper visa. There are no unproper visa, unless you count the ones via agents which f ake your requirements as fulfilled.
There is nothing "strict" about asking extra stuff that is not required by the rules. E.g. having been in Thailand before for 180 days and proof of that.
It was probably in a different group, but I saw it as well. Not sure, but I think I saw two posts from two different people at different locations in Thailand.