That's not how that works. The team calls before the pick is even made and confirms/comes to an agreement in principle on the bonus amount. That's why his family was celebrating before the pick was even made and knew that he was about to be selected at that very moment - they had an agreement in place with the Blue Jays to select him there. So of course he's going to openly say "I'm signing" after the fact, because he wouldn't have been drafted there if he wasn't signing for a pre-agreed upon amount. That amount that was pre-agreed upon is almost definitely going to be over-slot, based off of his leverage/standing in the draft. Now, we can more or less estimate the realistic amount that it would take to sign a HS pitcher of his ranking, and the numbers around that estimate don't really give much room for Holman unless there is some EXTREME savings in the remaining Top 10 picks. Keep in mind that the 19 year old JUCO kid we took in the 3rd round might also get a bit over slot, and we also took a HS OF as well who while unranked, might have also struck a deal for a marginally over-slot bonus (he did have a scholarship to Notre Dame).
I think you're reaching pretty hard in trying to find a "reason". The Rangers also drafted Will Taylor in the final round, and his realistic price tag is probably in the $4+ million range (hence why he ultimately wasn't drafted in the 10-20 range despite having the talent to go there) and the Rangers most certainly didn't save up $4+ million to give him. There were also other obvious "this kid is going to school" selections from other teams too; like Gage Jump to the Padres.