That's the primary benefit imo. The bigger action space is largely composed of non-strategic elements, at least in the sense of long-term strategies, eg micro and mini-skirmish tactics, that I don't think are as interesting. Ofc its clearly a conflict of interest, but my feeling was the most interesting aspect of Go/Chess is the AI making unintuitive discoveries that benefit the long-term. The human-collective machine is pretty good on its own at finding the shorter-term strategies; I don't think AI will make much significant impact in that space.
games as a medium to study upcoming real-world applications (eg cars), RTS makes sense; but as a medium to study AI beating humans, TBS is more appropriate (their ability to explore large search-spaces is far more interesting/potentially impactful). Studying both would be ideal ofc, but in a pick-one situation, TBS is better imo. But only RTS are even really viable atm, which is disappointing.
But ofc, there’s no tbs or grand strategy currently out there with a real tournament scene, so you can’t really count on the devs implementing an AI-API, or even properly balanced / bug-free (far more user-testing goes into sc2/dota2 than say civ, simply by virtue of its playerbase).