I'm curious too. The original developers were Humongous Games (Putt-Putt & Freddi Fish) whose other titles used the SCUMM engine. My guess would be Backyard Baseball was as well. I dug up this article that seems to go through the process but not in great technical detail: https://www.digitaltrends.com/gaming/backyard-baseball-97-in...
What I don't understand is why just grabbing the original game files and slapping it inside the SCUMMVM wasn't the whole story. That's definitely how the other Humongous Games were re-released on Steam a decade earlier. I just assumed Backyard Baseball had more complex rights.
IIRC, part of the order was for stuff like Steam achievements, upgraded assets, modern resolution support, other platform conveniences, plus other required changes to the game due to licensing agreements shifting, all of which made slapping it in a VM a poor fit.
I poked at the Steam Depots for some of these games for more information. A game like Putt-Putt ships the ScummVM under the GPL license and runs it with the original asset files: https://steamdb.info/depot/294661/
Backyard Baseball does not appear to be using ScummVM. https://steamdb.info/depot/3170541/
The .he* files also appear to be the original assets for the game. And then they built a Scumm interpreter to run the game which can talk to Steam to use leaderboards and achievements.