Headcanons trump logic in these people's minds. I was debating a dude on Reddit who said the exact same thing. His argument was that the Dioscuri wasn't light speed because that list of "anti-feats" listing shit from 2004-2006.
Yes, the "Anti-Feats" which is literally ignoring the faster and stronger feats listed in the series and ignoring that at the end of the day, F/SN's VN is literally about to be 20+ years old(I'm not kidding about the 20 years old bit, F/SN came out in 2004... it's not as old as people think) and is very outdated in terms of how Servants operate these days and is more closer to a lore storage at this point.
Headcanon truly trumps everything.