Yeah, the creator of this site is an ex-3S player, as am I.
Shielding in MBAC would incur you a damage penalty on your counter-attack, as well as a frame disadvantage. A normal held shield, I believe, was a 50% pro-rate, whereas an Ex Shield was 35%. You're correct in thinking it's similar to parry, but here, it's slightly different. When you shield in MB, it forces the other person's move to continue, as if they whiffed it. In 3S, you get immediate frame advantage, not so here.
Now, shielding got changed a bit in MBAA from MBAC. You can only hold shield in Crescent and Full, not Half. This is bad for people who hold shield (i.e. not Ex Shield which is tapping the shield button) since gaining meter in MBAA is different. It's pretty useless now. I'd focusing on learning how to use Dodge.
There's also Shield Bunkers (214D) which are Alpha Counters. 100% meter use in Half-Moon but I believe it's invincible. 50% in the others.
Some of this may or may not be unique to AA but anyways to correct what Hasuko wrote... I'm not 100% on my post either but for the most part it should all be true
Normal shields may or may not prorate more than EX Shield but at this point it doesn't really matter.
You will be at a frame disadvantage if you choose not to do anything since you won't be able to block and you'll be in shield recovery frames.
When you shield their move it is actually as if they hit since they still get cancel options ie they can double jump and block your shield counter, air dodge, air dash, I'm not sure how this holds up for shielding ground attacks.
EX-Shields are normal cancellable, normal shields are special cancellable only
Full cannot EX-Shield, only Normal shield so you must cancel into a special or shield counter (236D)
Crescent can EX-Shield or Normal shield but they do not have a shield counter attack
Shields are still useful but for the most part they are much less stronger and can be reacted to (pretty difficult to do so though) . The best is using a EX-Shielding in Crescent Moon and countering with a 4f jab (only some characters have such a fast jab)
Shield bunkers are not invincible but have a shield/clash frame at the start which will beat out their poke and hit them. However if they whiff cancel they will be able to recover and block/punish your shield bunker.
Half-Moon bunkers are much faster than other bunkers.