I see what the cop was going for, as Texas has a basic speed law. But I’m almost certain that does not apply to not driving fast enough if that speed is over the limit. It’s really for the other way around (driving too slow, or driving too fast in poor conditions even if you’re still under the speed limit)
It really seems like the cop just wanted to hassle you, was bored, having a bad day, etc. I’m sure you know most of Texas roads are a whole lot of nothing with no other drivers around so they’ll take what they can. Good thing it was just a warning! Generally speaking though I just ignore the limit and stay with the majority traffic speed. Cop can’t pull over everyone