Sadly, no, these aren’t true Monarch Underoos. Partly because they’re not really Underoos (“The closest you’ll find,” says the guy behind the auction) and partly because they’re total bootlegs. If you want them, grab them and the other Venture Bros. clothing the guy has up for sale before he gets shut down. (Update: Obviously if you’re reading this in the present day, the auction is long gone.)
Why Adult Swim isn’t making their own Venture gear I have no clue. And it just goes to show that for merchandising, somebody over there in the chain at Cartoon Network has no clue either. C’est la guerre.