It's not that much of a stretch. Groome was considered a candidate to go number 1 overall. Allard has a better 1-2 combo with fastball and curve but Groome has much more advanced third and fourth pitches. He could go through the minors quickly.
Allard has a better FB/Curve combo?
Groome: "Groome works with a 90-95 mph fastball that peaks at 97 with armside run, and he could throw harder as he gets stronger. Nevertheless, his best pitch is a curveball with power and depth that many evaluators believed was the best breaking ball in the 2016 Draft."
Allard: "His fastball will sit in the low-90s, with the ability to reach back for a little more when needed. But what really makes it stand out is the cutter-like movement to it and his ability to command it well. Allard has one of the better breaking balls in the Minors, a plus curve"
How exactly does that translate to "Allard has a better 1-2 combo with fastball and curve"?
Groome also hasn't already had multiple back surgeries, so his stock is justifiably higher right now.