Wireguard is just as fast as hardware accelerated IPSec on both the Edgerouter X and Lite. With Openwrt on the Edgerouter Lite Wireguard is ~2x faster than hardware accelerated IPSec on EdgeOS.
They don't give the IPSec parameters. Wireguard uses ChaCha20 for encryption with Poly1305 for authentication. IPSec can use many different combinations of cipher and authentication algorithms. If they were using AES+SHA256 then SHA256 would likely be the bottleneck. AFAIU, Ubiquiti routers use a crypto coprocessor (as opposed to AES-NI and SHA-NI on amd64), which means even if SHA256 was accelerated on the coprocessor it could easily still be too costly. But we don't even know if the MAC was accelerated at all. The implication that "IPSec acceleration" handicaps Wireguard is unfounded.
If those benchmarks were with AES+GCM, then that would definitely be surprising. But the safe bet is that they were using AES+SHA256 (or something other than GCM for the MAC), in which case their benchmarks are not surprising, simply misleading.
IPSec can also do ChaCha20+Poly1305 (at least, OpenBSD's stack can). Any serious comparison should also include IPSec using the same crypto algorithms as Wireguard.
Wireguard is just as fast as hardware accelerated IPSec on both the Edgerouter X and Lite. With Openwrt on the Edgerouter Lite Wireguard is ~2x faster than hardware accelerated IPSec on EdgeOS.