Corrected entry: When Thelma, Fran, and Naomi get up on stage and lip-sync to "Boogie Woogie Bugle Boy," they do a coordinated dance. In the real world (i.e., not a musical), three people can't decide to do a dance together on the spur of the moment, and somehow spontaneously know all the steps to a three minute dance.
Correction: It's a TV and movie convention, you just have to suspend disbelief otherwise 99% of the dance numbers in TV shows and movies would never exist.
At the 6:46 mark of the episode, Thelma tells Vint that Naomi, Fran and herself are going to pantomime dance as the Andrew sisters and has him sit down so she can show him the dance that they have choreographed when she is interrupted by Buzz and Sonia telling her they voted to have a "punk" dance with the group Medication.
This was a fundraiser dance at Edgar Allen Poe H.S. in Raytown. They were trying to "save" the school auditorium/gym. The punk band did not arrive, so Thelma, Fran and Naomi went on stage to save the dance. Prior to the students voting to have the punk band to perform, Buzz and Sonja, had given the idea of a 1940's USO Revival. T, F and N had already practiced this lip sync and dance routine w/the belief they would perform it. Also, 40 yrs ago who didn't lip sync and try to copy Michael Jackson?
It's a convention of musicals, where singing and dancing are fanciful or are depicted as performed by characters who are actors that have rehearsed a performance. It doesn't apply to this scene of a non-musical sitcom where the characters are aware that they are singing and dancing on the spur of the moment, and have not prepared for it in advance.
Correction: They don't decide to do the dance on the spur of the moment-earlier in the episode, Thelma tells Vint that they were planning on pantomiming to the record, which means they would have rehearsed the dance.