Free! Timeless Medley Kizuna is what it is: a compilation movie of the TV series. Before I watched this movie, I was expecting much more from it. I expected to see new content from the creators since their last work was High Speed! Starting Days that was released in 2015. However, the movie pretty much skipped through the first season of the anime and mainly focused on the second season (Free!: Eternal Summer).
Almost 80% of the movie was reused scenes from the second season of the anime, and a few snippets from the prequel movie. The movie's own soundtrack were also used in the
reused scenes and basically replaced the original music from the TV show. I did, however, liked the opening of the movie. If a person were to watch this movie before watching the 2 seasons or the prequel movie, it would lead to a lot of confusion to the viewer as the movie did not do a great job explaining the events of the prequel movie and the first season, as well as the characters. Aside from that, we only got 3-4 nice bonus scenes, although I expected much more. As a Free! fan, I kinda enjoyed the movie, but I expected more than reused scenes since there was a 1 year gap since their last work.
TL ; DR: Free! Timeless Medley Kizuna is a compilation movie that mainly focuses on the second season of the anime, mostly with reused scenes and new music that replaced the original in the anime.