Feel free to read
here. Plenty of evidence (some in-era and some out of era) for sub 75 agi slowing your run speed. But again, it's easily verifiable in-game.
-Duel someone and get to very low hp and check your agi/run speed.
-Wear str gear and have a shaman cripple you so your agi drops below 75 but you aren't encumbered and check your run speed.
-Roll an ogre and keep the base 70 agi without putting in the extra 5 points and run side by side with someone that did.
I'm not going to go to the effort of making a video demonstrating these. If you don't want to believe it and you don't want to test for yourself, that's on you.