This is what happens all of the time. While each pick comes with an assigned amount, 90% of picks sign under that amount, and especially near the top of the draft. Brooks Lee signed last year for less than slot. Noble Meyer got $1.5mm less than slot and the Marlins gave it all plus more to Thomas White. Paul Skenes signed for $500K less than slot (while getting the highest signing bonus ever).
So, yes, I think it plausible that the Twins could sign Jenkins for $6.5mm, which is $600K less than the $7.1mm slot, and give that savings to another prep player they drafted. I don’t know if it will happen, but it’s entirely plausible, because that’s how the draft works with the fungible draft pools.