The amount of functionality does not compare to other table/grid frameworks. This library is not "just a JS plugin for tables", it can do a lot of things. If you try to rewrite it yourself you'll need a lot of man-days to do that.

On the other note:

What if you fork that repo and change something small, can you use it for commercial purposes? Or is it stated that any modification MUST use the same license, and you must pay the original creators?

You can't fork the current version and use it for commercial purposes, regardless of what you change, but you can fork the previous version and keep using it under the old license.