I'm at a loss as to how alphabet agency can determine a weak passcode vs strong passcode was used. how does a pin get stored on the phone? surely, not plain text of a 4 digit pin. if they do any encryption to the 4 digit pin, how would it appear any different than a significantly stronger passcode?

The grandparent post was about determining the complexity of a PIN/Passcode by watching it being entered - more screen interaction = more complex.

It uses a different screen. If you have a 4 digit pin, the entry screen looks a lot like the phone dialer, with the numbers 0-9.

If you have a stronger passcode, you see a full keyboard instead.

The prompt is different based on the type of code you use.