I think it was mutual to a great extent. Disney knew that Sony was desperate to rejuvenate their Spiderman movie franchise, and at the same time, Disney wanted the extra bump for the MCU they knew would come with adding Spiderman to the MCU and developing him as the "heir" to Iron Man of sorts.
Plus, Disney owns the merchandise rights, so a healthy Spiderman movie brand is in Disney's interest either way.
Now the problem is that Disney doesn't really need Spiderman (given they've just gotten back FF/X-Men from Fox) to the extent of giving up a full MCU movie slot every 2-3 years to Sony so they're asking for much more. They need to be releasing much higher quality movies than Venom (despite its huge payday) to make that work though.
At the same time, Sony's realized that maybe they can build their own Spiderman Marvel Universe around Spiderman/Venom and whoever else is getting movies or tv shows over the next couple of years. That's especially true given how young Holland is. He could play the role for quite a while if he wants and build a whole cinematic universe for himself.