Their wanting to control the development is only contingent on their livelihood not being threatened. I may be wrong, but I don't think they'd mind people developing an alternative forked "Dwarf Fortress" if it meant a seven figure annual income for them to keep working on whatever they want.
In the situation of their opening the source code, I don't even think they'd be even "administering" the game development: rather, a team of trusted volunteers would probably do that, fix the bugs, accept patches and merge the branches accordingly while Tarn would just keep churning on whatever features he likes.
Of course this all raises the matter of a guarantee of said annual income or team of kind-hearted volunteers, as real-life constraints exist, drama in open-source communities is a thing, and people working for free do have a habit of disappearing with no notice or justification. But the more time passes and the community shows unwavering support, the more confident the Adams brothers can be. I could understand their reservations 10 years ago when the project was still pretty much in its infancy and Tarn's reputation wasn't that widespread, now he's carved a solid enough niche for himself that he could take more liberties and be more trusting imo.
In any case, the Steam release will prove crucial in the direction of future development. Ideally it'd be a resounding success, set them for life, so that they feel comfortable opening up the source or parts of it. In the worst case there will be backlash as new players stumble upon the game's quirks and unfinished state and performance issues, requesting refunds and forcing Tarn to delay his pet features for more bugfixing and polishing, which is tedious work and kinda demoralizing. More likely, it will be something in-between.