we use a buck-boost charger circuit which allows charging from lower input voltages.
Lower cost designs use a buck only charger, which will only work with 20V power adapters.
This saves a little cost, as you need less switching FETS for the charger circuit on the mainboard.
The 5v bit seems to physically work but it currently chooses not to, you can trick the laptop into doing it but that doesn’t have a lot of real world use. Hope that gets fixed at some point.