What makes you think so?
The devs said so. Check r/Suyu, that seems to be where a majority of the updates are being posted. I think there was a link to a pastebin post somewhere there as well.
The SDK mentioned was first party, presumably leaked but I'm not completely sure. And yes, that means it would be present in every other fork as well.
Edit: here are some of the links I'm talking about:
https://www.reddit.com/r/suyu/s/TqSWDlnsGs
Edit 2: worth noting that the "founder" (as they call themself) still wants to continue on the project but I believe a majority of the devs left.
Edit 3: I found the archive link from someone on the Yuzu team showing they had access to a leaked switch SDK: https://web.archive.org/web/20210114104638/https://twitter.com/Slashiee_/status/1349557173970341890
I don't know how much of this evidence is real but if any of it is they're going to have a much harder time finding devs willing to contribute to Suyu, even if development does continue.