The publication of that definition is what caused us to use the word “open-source” in our vocabulary. And the first sentence in that definition is “Open source doesn’t just mean access to the source code.”.
When I talk to our legal team at work and tell them that a library is open-source, I’m effectively saying to them that there’s no legal restrictions on us using that.
Mere access to the source code does not offer that. You could be granted access to the source code and not even be allowed to modify it, as you suggested to OP.
As far as I can tell, this is the case for Grayjay. So, yes, OP can modify it, assuming they don’t get caught doing so.