I feel Chelsea don't really want to sell Jackson, it's more like, if for any reason for them to sell him, then the amount must be something significant, so that's why his release clause is that high. Nobody has officially come for Jackson, just like you've said, we've been hearing only rumours of Manchester United picking interest which so far with the signing of Bryan Mbeumo, it has shown that they weren't serious, however I believe Jackson will be an integral part of Chelsea squad next season as the club has not shown much sign about letting him go.
I think Manchester United's interest in Nicolas Jackson will also remain just a rumor.
Chelsea is unlikely to let one of its best players join a rival club.
I also agree with your argument that when Chelsea set a high price for Nicolas Jackson, it meant Chelsea was simply looking for a chance to make money from the sale.
In fact, Chelsea set a high price to prevent other clubs from being able to sign Jackson and keep him at Chelsea.