I haven't been able to replicate this using P1's shield input, but it did work with P2's. I found you can't do it against Sheik if you're playing as SoPo, tried it with Yoshi and Sheik shielded. In training mode I have SoPo vs Zelda. Shield input for P2 makes the computer shield even if you don't have the AI set to control. I set it to control, transformed to Sheik, and set it back again. Then I did shield input on P2 and Sheik didn't shield.
I tried changing from stop->control->stop on Zelda then shielding and it still shielded. This happens for most characters in training mode, I've done it against a ton of other characters and I assume it's a little easter egg for shield pressure practising.
edit: this also works in vs mode