I actually found this worrying, since it means they are not committed to make NextJS work nicely, quite the opposite they now have incentive to not make it happen
Even if Vercel was built on Deno, I don't see them open sourcing that. Everything that manages distributed cache, edge rendering... They'd probably rather keep that closed and available only for Vercel customers.
The NextJS team should be working on making NextJS run better with Deno, not the other way around. If the NextJS team doesn't want to do the heavy lifting with that support, thats on them. I think its perfectly reasonable that the Deno folks have an approximate alternative to NextJS. After all, NextJS won't use native Deno idioms for example, it'll always be tied to Vercels' proprietary cloud infrastructure.