Basically, WB asked Marc Guggenheim to include him to build up goodwill and keep him in the public's mind ahead of the 2022 movie.
https://variety.com/2020/tv/features/crisis-on-infinite-earths-ezra-miller-cameo-marc-guggenheim-interview-1203467952/
https://variety.com/2020/tv/features/crisis-on-infinite-earths-ezra-miller-cameo-marc-guggenheim-interview-1203467952/