I think perhaps one sort of semantic/slight difference is that Holler is hyper-local. If you live in LA and your friend lives in London, they obviously aren't going to the local farmer's market with you. I'd say the vast majority of my WhatsApp friends aren't local (I'm based in the States.)
BUT I can totally see that being an easy way to do things if your local friend group is on WhatsApp!
One of my questions would be whether anyone (strangers?) can see that or not. And again this is a US-centric concern, but one of the things I think a lot about is ICE and the government tracking non-white people, and so if there's any semblance of a public "I'll be at X place at Y time," then I'd rather not make that public. So Holler solves that.
But yeah, that's a creative use case for WhatsApp I hadn't considered! Very neat. Thank you for explaining.