I recently had a director reach out to me about joining their team to work on rebuilding an ad system from scratch, so unfortunately your fear may very well be valid.
https://www.roblox.com/catalog?Category=0
But I seem to recall in the past there used to be some regular ads. For example see this archived page:
https://web.archive.org/web/20120427192637/http://www.roblox...
which has this embedded iframe:
https://web.archive.org/web/20120308000433if_/http://www.rob...
which has Adsense javascript embedded in it.
It was honestly quite jarring and this new approach is better for developers on the platform, as they can reach way more people.
they have an extremely high revenue business model off actual customers, so they don't really need to do advertising, there's plenty of other ways to get more profitable.
for example, they're at the scale where they might be able to do what Dropbox did by running more of their own infrastructure to save big.