It seems likely that commentators will be top players who have been eliminated from the tournament; thus it's not possible to specify them in advance?
Moderators will decide about the commentators. If a player is unable or unwilling to have their match streamed or commentated by the selected commentator, they forfeit the match.
If the moderators want the authority to make people forfeit matches, it's pretty good practice to do it as objectively and transparently as possible. You don't want to get into the situation where this was the rule, we get to the QFs and the mods decide "OK we're having AdamH do the commentary" and all of a sudden people would rather forfeit the match. It could easily be seen as a move done by those dastardly evil mods -- "MWAHAHAHAHHAHHH I want Joebob to lose the tournament and I know he hates Adam, so I can make him forfeit by telling him Adam will commentate!!!!!" when it could have all been prevented by just listing all of the commentators before the tournament started. If I was a mod, I would want that rule to be written before the tournament starts so that I could just point at the rule and everyone's life is easier (except for people who join the tournament without reading the rules first. There's no helping them by making better rules, sadly).
I am just posting to say though, that if you are in a Magic: The Gathering Pro Tour, and make the quarterfinals, the match will be filmed, and there will be commentary, and you will have no say in any of that. And they have had players eliminated in earlier rounds do commentary, thus not possibly specifying them ahead of time. And that has all worked out, no-one ever forfeits to avoid that.
I of all people don't mind being filmed, but I think it would be a good idea to explicitly state it in the rules if the organizers want to be able to kick someone out of the tournament for any reason, especially if they necessarily have to make a deep run to get there and there might be higher scrutiny.
One idea is to have a pool of people to choose from, and only choose people who have been eliminated so far.
But yeah in Magic, do they really have people who played in the tournament commentate? I thought that with any competition of that kind of scale they would have dedicated commentators. It would seem weird to me but I'm not educated on the issue.
But as for this specific tournament, yes the commentary situation is a significant concern of mine. It comes from the fact that the TO and other people I suspect are involved in the tournament (organizers, but actually not any of the moderators, which is a big deal) have had a significant issue in the past related to commentary. Given that that's happened, regardless of my personal feelings about it, it seems like a smart idea to at least say something more specific in the rules to prevent a similar situation from getting bad in future tournaments that they run, like this one.