It doesn't exactly. But a grout that can resist that much flexural pressure ain't gonna be that flexible is it. Its rigid, its higher density and solid as ****. Grout needs to have a lower denisity and capillaries in it to allow flex and limited movement. Epoxy doesn't have that.
Like i said, there isn't a test for actual flexibility defined by any standards, but the flexural strength is a good indicater.
Don't have to take my word for it. Ask Bal, ask Mapei, ask Ardex..... Or do your own test.
Rule of thumb i was taught and have stuck to for 20 years:
CG1 - not flexible
CG2 - flexible
Epoxy - flexible but nowhere near as flexible as CG2