Can this be used on shower floor tiles?

This is called an epoxy grout. Some call it resin grout I guess. Epoxy grouts are pretty much the toughest grouts out there. Chemically nonreactive, super strong, Probably waterproof, since the internet says it is. They say it's no replacement for proper waterproofing though. So still do that in case you get a crack down the road or something. Given all that, I'd say there's no better grout for a shower floor if you don't mind epoxy's drawbacks, being the 5x + cost, extreme messiness (count on it to get on the floors when doing walls) and potential to ruin tile within 1 or 2 hours if not cleaned off (dries fast and super tacky,) and difficulty to remove it if you don't like the color. But if done right, you can feel pretty good about this stuff after. For the love of God wear gloves.

Yes it can be used for shower floors. You must allow 24 hours for it to cure before using shower again.