Is it possible for SMEs to directly edit files on SharePoint Online without a RoboHelp license?

Without diminishing anything the others have said to this point, it would absolutely be possible. Good idea? Unless someone really understands HTML, probably not. But possible. 

If your organization goes this route, I would be sure that versioning is set up on your SharePoint Online site. By default, it should be. This would allow you to more easily roll back changes to a 'last known good' version if they really did screw something up beyond repair.

@Stefan Gentz I considered answering this question that way, but the risk of somebody messing up your source outweighs somebody screwing up the output (at least imho).

If they goof it [the output] up - no big deal; make the changes desired in the source project & republish again - low investment. The reverse scenario is a lot more weighty - somebody's messed up your source - can it be fixed? do we have to restore the whole thing from a backup? did we lose anything by doing that? See? much bigger investment there.

So to be safe - let them tinker with the output & let you know what's to be changed - at worst, the situation you could be in is not getting all their precious "emergency" changes into the source project.

I second @Jeff_Coatsworth, here. However, as so often: It depends 😉

After all, the published RoboHelp topic itself is "just" an HTML5 file. So, I tend to say: Everyone who understands XML and HTM, has some good feeling about being "responsibly careful" towards the code of others, and some "normal level of intelligence" should be able to do such last-minute changes. And for sure with a quick briefing and syncing among each other.

Your chances of somebody messing up your project are extremely high - let them play around with the published HTML output, not your source RH project files.