... this takes place long before that happens; here, Undyne's a kid/preteen in training and Asriel's literally a baby. All'a that hullabaloo occurs when Undyne is captain and Asriel's a kid ... I figured that'd be an easy assumption ._.

This is meant to establish that Undyne was at least familiar with Asriel but didn't get to properly meet/get to know Toriel until all of the drama.