Well, I had nothing to post about the past days, but I did this control: I signed in to Facebook, then went to Instagram.com and I was NOT automatically signed in to Instagram. The last time I did this control, I was automatically signed in, but this time I wasn't.
On my phone idem dito: I signed in to Facebook, then went to the Instagram app, and Instagram wasn't signed in, it asked for my password.
So no longer being signed in to Instagram when being signed in to Facebook, gives me the impression that disconnecting the both accounts has worked.
I guess when I post to Instagram next time, I'll be 100% sure, but the test I did now seems positive!