the dt minimum value can be fixed to fit a 60 fps display so if the game goes under 60 fps, well, it will get slowed down.
Yes, but a maximum can't be set. The only way would be to set the minimum to something like 600 so it has no chance of ever varying on faster displays. It's still going to run faster on faster displays though.
using dt is still important to ensure that the result will act the same regardless of the framerate (so a 144Hz display or a 60 Hz one will still be calculated the same).
Having a fixed framerate completely alleviates this issue, and there's no need for any dt at all. The game and rendering rate are fixed according to system time. that's the whole point of a fixed framerate.
But apart from performance reasons (executing the logic and rendering at a fixed 30 fps is not as costly), if it is done well, dt ensures that as long as there is no lag or freezes, the game acts correctly and is as fluid as it can (since the movements aren't framerate dependant but time dependant).
Exactly what isn't wanted, time dependent movement. For most games and applications it's fine, but sometimes you don't want that. It's good to give people control of their game's behavior. Forcing them to use V-sync and the corresponding logic rate isn't something an engine should do. Don't get me wrong, most games SHOULD use V-sync + dt. The game I'm working on now (not with Construct) uses V-sync and dt. But there are times when you don't want or need it. Old games did not use any kind of dt and thus were perfectly deterministic. Some people want that.
Also I never thought of a single case when anyone would want to use every X seconds with such a low value as 1/60, like why even do this? I don't get it, if your dt implementation is correct you simply don't need nor want that, perhaps I'm missing something (it's early here).
I'm showing you the only workaround that you can do in C2-C3 to acheive a fixed framerate, but I explained the problems above. The event sheet running at vsync rate makes precise timing impossible, resulting in very jerky movement. It needs to be a runtime option to work properly.