There's a level of performative solemnity and indignation expected of politicians speaking about national tragedies, and Muslims about 9/11 in particular. When someone like Omar refers directly to an event like 9/11 with plain language, people (even ordinarily well-meaning ones) will immediately jump to the conclusion that anything but the compulsory refrain of explicit condemnation is implied disrespect or disloyalty, even as the expectation of vociferous patriotism is part of the discrimination she is criticising.
I won't pretend she got that message across to people in the best way, but it's important that people realize that topics like these are fraught with taboos, emotional landmines, and misunderstanding, and it's both common and necessary that people say things that won't sound elegant to everyone in order to move the conversation forward.