Space-faring electronics aren't exactly cost-sensitive - the cost of a cluster of crypto-accelerated CPUs or rad-hardened FPGAs is peanuts compared to the human and launch costs that go into these satellites.
Issue is the satellite was launched 10 years ago with 20-year-old tech. So, calculations of today may not be applicable on them.
Wireguard uses ChaCha20, which to my knowledge neither has nor requires HW acceleration to be fast.
It's faster on CPUs without dedicated hardware than AES, but that doesn't mean that it's faster than fixed-function AES hardware.
> However, the software performance [of wireguard] is far below the speed of wire.