Ray ask Sara if there was a crossover and if they were invited, a breaking-the-fourth-wall joke as the Legends weren't part of the Elseworlds crossover.
When Ray and Ryan mention their respective names for the universe entered by shrinking, Ray points out that there would be copyright issues regarding "Microverse"; this is because it is competitor Marvel's name for their universe entered via shrinking (Legends and other Arrowverse shows are of course DC).
Lois told Clark to get home as soon as possible, because of something with "the boys". A reference to the upcoming Superman & Lois series.
J'onn wears a suit that is similar to the one he wears in the comics.
Rene says to Mick "Rory, huh? We had a Rory once on our team," refers to Rory Regan, the superheroic Ragman.
Marv Wolfman: The "Crisis on Infinite Earths" creator appears as the man who asks Flash and Supergirl for an autograph. He only introduces himself by his first name.