He is full of caca.

Call the grout manuafcturer and ask them what they think about that idea. They will tell you you now have no warranty. Don't you think if "glue" made epoxy grout better someone would have introduced the product already?

There are liquid admixtures that are milky and look like glue. https://mapeihome.com/flooranddecor/product/ultracare-grout-maximizer/ is one from Mapei. I would use the manufacturers product. I hear epoxy grout is difficult to work with.

Admixtures are meant for standard cementitious grout. Not epoxy. You would create a witches brew of failure. And, they do very little for cementitious grout to begin with. Better to buy the better quality grout from the beginning like Ultracolorplus FA, or Flexcolor CQ.

Sorry I think he was referring to add some of grout additives (grout maximizer as @Seabornman mentioned) to the traditional cement-based grout, so that it would make the grout stronger and more stain-resistant and better waterproof?

Found this youtube video: https://youtu.be/SkWqM7QNtNg?si=5uUwzjlG6UyZNyb-

Also, have anyone here have any experience about this? With this approach, how would this compare to use real epoxy grout, such as "The Tile Doctor's Starlike EVO Epoxy Grout"? Thanks!