Update:

So I swapped the battery and my bike still couldn't start. Apparently someone faced a similar issue in the Ninja 400 forum but no one came up with the solution, and a video of the exactly same scenario was posted:



ChatGPT is saying it's either a weak new battery or a starter on it's way out (which won't start unless the battery is perfectly charged). The latter seems a possibility given my circumstances. I will try to start my bike again and if it still fails, I will push start my bike and bring it to a mechanic.

Edit: Seems like there's a common issue about push starter for Ninja/Z400, where the contact gets rusty and requires cleaning. I might also look into that.