(Note: Solved now, but keeping this post here for future reference. Text is edited from the original slightly)

The LRT for Penta+Deln's Co-Op Any% 2 player run shows an LRT of 1:21:29, but I did not know how they managed to calculate this.

LRT (Load Removed Time) pauses the timer during loading screens to make it equal across everyone's hardware. The timer continues running when you're in the main menu, however. I think we all know this.

Most Any% runs can be taken as an example for seeing it in action:

11:59

33:08

Note that Co-Op runs have an exception to this LRT method.

When in the main menu, the timer resumes like in any other category, but when going to the Lobby for co-op (letting player 2 join), the timer continues here too:

32:31

Penta+Deln's, as well as mine and Molly's Co-Op runs all use LRT.

At the end of Penta's video, the LRT timer shows 1:25:18, whilst their submitted time on the leaderboards and also in the video, claim it is actually 1:21:29.

1:29:03

This run uses LRT like normal, so what makes them think they achieved this speedrun time? I figured it out after trying several methods:

Penta's time of 1:21:29 is achieved by taking his video, and subtracting all time spent in the main menu + lobby, from his LRT time.

***(**his timer only shows one decimal, so I started framecounting from the first frame the decimal number went up, to the last frame it does in the loading screen)

Framecounting the total amount of time that the timer is ticking in the main menu, during his run, yields a total of approximately 13887 frames, or 3:51.45 minutes:

It is roughly a second off, due to his timer using decimals, and if you pay close attention to my co-op run video, you will see that the timer stops for a few frames when loading the lobby menu. This would take a few frames out of the total framecount with this method.

Re-timing My+Molly's run yields this result:

A WR! Now you know how co-op time is calculated

It would be best if this was mentioned in the category rules.