My results disagree with that. I’m running a FlightAware external antenna indoors → 1m CLF200 coax → Uputronics SAW filtered preamp → Pro Stick Plus → RPi, all sitting indoors on a windowsill. Adding the preamp increased positions and aircraft and range immediately. This arrangement is a very quick and convenient way to improve things, and the preamp is available to improve things further if/when the antenna goes on the roof.
I agree it’s not a panacea for a bad setup, but it’s certainly useful beyond the example you gave.
With the preamp in place I’ve improved aircraft and range even further by changing gain from default -10 to 49.6 as detailed here. I found that positions actually went down in some cases, while aircraft and range simultaneously went up.