In theory, it is possible to complete these in under one second which can occur if no one else is in the game. Are there any rules against submitting the time in that situation?

Yes, its possible to start a new game in survival and spawn and be immediately the 1st place in the scoreboard, but its kinda random thing to happen, so idk if i should delete that category, also that category is a misc. one sooo, idk, also for your question, no, there isn't any rules against that

This can happen for other modes as well, like I have seen it in pretty much every mode that has a leaderboard before, particularly FFA and maze. So that's why I was wondering about those as it makes it possible to complete the leaderboard and 1st place as soon as you spawn.

Sooo... well i still don't know what to do.

Well its up to @Noice, many games such as paper mc and seterra have many 0.001 sec runs

If you Noice don't feel like that fits the theme for diep.io add a rule and reject that kind of runs

@ninjahuman , @WhatsFrog i think i'll just delete that category

I think that may be a good idea.