It's an opportunistic play that didn't find an opportunity. Pretty reasonable to return the money.

Ackman still has a reputation to protect, so he needed to look for some sort of reasonable deal with good flippability (that's the Latin term). That's gone now with all the air deflating from the SPAC market. A SPAC play would not likely be the kind of thing he'd buy and hold for ages, especially as everyone knows there's some serious adverse selection going on concerning which firms will go for a SPAC.

There's a fair chance his sales guys are able to simply move the investors over into his main fund or some similar vehicle, so it's not actually such a big deal for him financially. Plus you get kudos for not doing something that might be seen as reckless or profiteering.