Realistic Possibilities:
1. 100% clean - Floyd is heavier and hit Tenshin behind the ear, throwing off his equialibrium for the rest of the fight (making him do the silly flopping)
2. 100% clean, but Tenshin playing games - Tenshin wanted to sell some knockdowns and make a valiant comeback to make himself look like a hero, it backfired and his corner threw in the towel.
3. 50% clean #1 - Floyd didn't know it was fixed, Tenshin/Rizin did. For whatever reason, Rizin wanted Floyd to win by KO, and worked it out with Tenshin.
4. 50% clean #2 - Floyd didn't know it was fixed, Rizin didn't know it was fixed, Tenshin had Yakuza "encouraging" him to lose in the first round for gambling purposes.
5. 100% Fixed - Floyd only agreed to sign the contract if some backstage work was done to protect his legacy as well as prevent him from taking any damage.
Any of these seem possible to me. I don't think any of us know for sure unless something comes out substantial. It's hard to analyze just from viewing it.