I think your research is mistaken. It is quite possible to place Google's adsense code in a Nabble forum. It is just a matter of adding the code to an appropriate NAML macro.
Those who do that will normally pay to have a "Premium Site", i.e. one that does not show the adverts that Nabble will insert.
There are a couple of topics that should help if you search this forum for "adsense NAML"
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.