After retesting I was mistaken about this. When I tested it the first time, the opponent's grab was whiffing, which is why my HP Beat Extend beat it. I tried it again setting up the dummy to reversal with Merry-Go-Rilla and with Ultimate Showstopper and those both beat it. Thanks to @
Broken Loose for pointing this out to me.
Actually in general Beat Extend seems kind of garbage as a reversal? Or even just as anti air. I was trying it out last night and it was just getting beaten out by random normals. I was mostly using the HP version, then later in the night I started switching to LP version. Maybe the MP version is the secret, I didn't try that one much.