I wanted to start speedrunning on Hollow Knight, but I only have the game on Switch and I don't know how to record my gameplay or if autosplitting is possible. Can somebody help me ?

autosplitting is not possible on console, to record runs you can use a capture card or in last case record your screen with a camera

Is the timer not needed for submitting a run ?

No, since it's RTA you can deduce the time from the video itself. Check the rules for when time starts and when time ends

Sorry if that's a stupid question, but are the rules on the discord ?

The rules are here on SRC https://www.speedrun.com/hollowknight?h=Console_Runs-Any-Switch&rules=game&x=7dg9e9gk-38do641l.21dd2rg1-p8591z0n.21ggxyo1