Peak pricing is indeed higher, but also much rarer. Prices of $2/kWh are reached extremely rarely, so the VPP in this scheme might only be operating once or twice a year. The calculation in the GPP about payback time for the powerwall assumed you could get a lot of cycles in, but at these elevated prices that seems unlikely to happen. Note btw that the $2 is flat, so even if prices reach $9/kWh you'd still only get $2/kWh back.
I notice that the article does not include any expectations about how often the "events" would happen btw, even though the utilities would surely have enough data to at least make an estimate.
The payback calculation was a direct reply to the concern that VPP use will wear out the battery too fast. The only reason it was done was to show that if VPP would trigger often enough to meaningfully harm the pack, it would also pay you more than enough to replace it.
In practice, VPP will be rare, and thus the payouts will be low. But so will the harm caused to you by participating, assuming you already own a suitable system.
I notice that the article does not include any expectations about how often the "events" would happen btw, even though the utilities would surely have enough data to at least make an estimate.