The difference is whether you both want to get penises involved in the festivities at some point. Or vaginas, if it’s two ladies.
If one person wants to get freaky and the other person doesn’t, that’s by definition not chemistry. If neither person wants to smash uglies with the other, that’s friendly chemistry.
I’m lucky enough to have a really good lady friend where there’s no romantic tension whatsoever. It’s the best. Most of my female friends, there’s at least a little sexual tension. That’s fine, I wouldn’t want to change that with those friends and I know how to handle it. But this one friend we could just talk hours and hours and there’s no furtive looks like, “man I wish we were both single.”
Just fun, interesting discussions. That’s what non-romantic chemistry looks like.