Yes, that seems to be the case, at least according to theory. However in my experience that parasitic diode does not seem to cause a problem emulating keys in a battery operated remote. Maybe the low voltage doesn't interfering with such a small DC offset since the key scan signal is low. All I can say is when using NPN the polarity only worked one way but with MOSET it worked either way.
JFET are truly bidirectional but the requirement for negative turn off voltage will be trouble here.
In this case the signal is most probably digital input type not matrix so 2n7002 definitely needs to have source, like emitter of a bipolar, to ground. I should go back and make that clear.
BTW those MP1584 modules are crazy cheap. About 50 cents each on Ebay ( https://www.ebay.com/itm/5PCS-MP1584-3A ... SwX~9dUQFx
) or half that on Ali. Very useful but using the Pi supply is definitely the way to go for replacing battery in a remote like here.
It's possible to run the Pi itself off one but only lower power models with no additional attachments. 3amps is typical Chinese humor. With that tiny inductor maybe half that or less in reality. Worked with my Pi3b+ and Zero but got 'lightening bolt of death' with 3b and 4b under heavy load. Better to spend a few cents more and use the LM2596 ones to be safe.
The small red one jondallimore and rin67630 discussed in the other thread does actually put out over 2a and has much lower idle current but costs almost a whole dollar! lol