i said modern grout, yes? which would all have an epoxy element to them.

also, i said "theoretically” waterproof originally. didn’t think that the minuscule amount of water absorption was material to the conversation.

modern grout is waterproof

grout is theoretically waterproof and rigid, where caulk is waterproof and flexible. the flexibility is because it will go through extreme swelling/shrinking cycles and over time will separate from its surface and eventually fail. grout does not have this problem and is more permanent.

re grout is the way.

alternately, you could clean it out and fill with epoxy. get yourself a grout removal tool, and get as much out as possible. then put tape on each surface, leaving maybe 1/16 from the tile edge for epoxy spillover. you want to overfill just a little bit, and when the epoxy starts to get tacky (but before its fully cured) you peel the tape, and trim up the epoxy overfill with a razor blade to make a nice edge. ezpz