I'm sure LFM could tell us more about how these things work but I can't imagine it's likely that Onstad would actually end up doing the voice.
It all depends, really, on the approach they were taking.
If they're doing it for the networks, yeah, they would probably want as many semi-"name" voice actors to be on as possible, to create mainstream visibility or at least visibility in a lot of difference niches at once which is... kind of the closest thing we have to niches these days.
If it's on something with a shoestring budget, though, like Adult Swimmy sometimes they like having creators voice things because then they don't have to pay them anything more than the minimum because they're already getting their slice as writers or producers and it gives them a certainty that said voice talent won't jump ship over a raise because, you know, they're otherwise attached.
Plus having the creator do a voice or two helps when you put them on a talk show or a comic con panel because... hey... the casual fan who doesn't care who the writers or producers are but knows the voices can be like "hey, that's the voice guy"! The horrible fuck*ng goddamned awful Seth MacFarlane shows are all about that shit and so is the sparkling amazing best fuck*ng show on television ever show The Venture Brothers.
They might believe that having the creator as a voice actor would lend a certain stamp of authenticity to bring along the cult audience.
So depending on the outlet and the outlet's marketers, some would consider the creator doing one of the main voices a moderate plus just as some would consider it a non-starter-level minus.
The fact that the other people on this sample are recognizable character actors/voice actors suggests to me that if Onstad wasn't at least hoping to voice Beef, they would've just gotten another semi-known voice actor for the role.