"No, since the game simply gives you more tasks."

What even is that argument? It's not as if as soon as you finish all tasks, they instantly reset. There is a screen that pops up as soon as you finish all the tasks. What else do you need for it count as an ending? Do you need a cutscene and a credits page with inspiring music? What matters is that there is a clear interruption between the end of one game and the start of another.

That just seems like the same as saying that any story game shouldn't be speedrunnable "because there is the option to just start another game".

If you stick with the argument that it is short/trivial, I'd understand; depending on the maps, the runs would be around the 5-minute mark (even though there are shorter speedrunnable games, all it took was a quick google search - https://www.speedrun.com/speedrunning/thread/r1392/1#76yj3 ).

However, the claim that there is no defined ending simply comes across as subjective and not a good justification overall.