If you really don't think that the console would let us know then it needs the Joycon's battery, either with a notification or just the classic LED (For example, the 3DS has a red LED when it's low battery, and blinking red when it's super low. When Switch starts blinking red, you know it's using the joycon's battery) then I really don't feel like having a conversation with you. You're just assuming what you want to assume in order to somehow turn this into bad news lol
It's not just about a notification, it's about balancing the battery left in the joycons with the battery left in the tablet and opening the charging circuit only under certain conditions. It could be possibly a pretty weak spot if it doesn't balance this properly.
You're just assuming that the joycons will have 100% or so battery, while under the use cases presented by Nintendo that's not always the case.
What if you just played a lot with them detached and the joycons have even lower battery left than the main unit, while main unit is under the required threshold to be charged? You practically have still some battery in the tablet and lose part of the controls.
I'm not trying to turn it into bad news. I'm just saying that it's not very likely to happen. In a way I try to avoid to turn it into bad news once this is not confirmed.
Edit: in the end trying to charge a 4300 mAh with two 500 mAh batteries might not be that efficient. Or fast enough.