I think Grade A prospects is just too high a bar here when we're talking about Toffoli. Heineman is a solid piece with some intriguing tools and solid production in a good pro league, and we also got a 1st back in the same deal, a blue chip is just never going to be on the table for this kind of player.
He's a good player but we didn't trade Jack Eichel here, he's a solid top 6 winger that's currently being used on Calgary's 3rd line because he's not definitively better than Coleman or Mangiapane, and they have two clearly better wingers in Tkachuk and Gaudreau. We got a 1st, a recent 2nd round pick which is more valuable than the pick itself given you're saving 2 development years and have more certainty with the pro-readiness of the player, that's good value for a 30 year old top 6 winger.
Yes it is silly that the value gap between Chiarot and Toffoli will probably only end up being Heineman and a 5th, but that's due to Chiarot's value being overinflated, not due to the Habs selling too low on Toffoli.