While the source engine has historically been used for FPS games, that doesn't mean it is restricted to only FPS.
It doesn't but it's certainly not RTS-friendly either. There are too many problems with making an RTS game at this point. I know 2 RTS projects that were started but were abandoned due to the ridiculous amount of limitations.
Unit
box-selection compared to Blizzard's games, handling many units and unit groups, spawning units is an unnecessary mess, pathing and navigation is certainly not efficient. Server ticks at 30 fps "only" (even if SC2 is 16 which is enough but the scales are different between these engines).
On top of that, the majority of the community has external resource limitations, there are only 2 groups in the entire scene that make their own In-game assets, the rest is either using default assets or ripping off other games (such as WC3) which is a free-ticket to a DMCA takedown. Dota's default assets don't have a lot of buildings (basically 4 towers, one barrack and one ancient, that's it). Best thing you could do is paint them differently and tell the users to use their imaginations because your quality bar is with the rest of the workshop scene right now.
And these are only the major problems.
The conclusion is: Making a low-quality game that
kinda looks and plays like an RTS is doable (if you don't care about gameplay, smoothness or quality).
Making a high quality and smooth RTS right now that meets realistic standards is almost impossible.
