Basically it does not need dedicated hw acceleration because it can use generic vector instructions to reach similar speeds. I wonder how true that is though.
[1]: https://www.wireguard.com/known-limitations/#:~:text=WireGua...
Taking the DJB crypto path gave Wireguard some subtle advantages to implementation ease-of-use that are almost entirely overshadowed by the difficultly in building a new, secure cryptographic protocol from scratch regardless of what algorithms you're using. The tradeoff was that there are plenty of places it will never be used due to standards compliance requirements which as you point out also has significant implications for efficiency in hardware.
Wireguard is cool. I think very little of that coolness has to do with the DJB vs NIST cryptographic choices. And taking the DJB path unnecessarily limited the impact of its coolness at least for now.