If you receiver is hearing an ambient noise level that is above the
internally noise floor within the receiver then an antenna tuner will
only bring that ambient noise leveI up.

True.

If you were hearing Radio Fly
Speck against a background of static and weaker stations, then
adjusting an antenna tuner will increase the signal strength of
everything including RFS. It won't magically lift just that single
station from the mud.

True.


Said another way it isn't possible for a tuner change the difference in
signal strength between the desired signal and all the other signals
that are competing for your ear on the same frequency.

True. However you are missing one more possibility. That is the impedance
mismatch
between the antenna and receiver is so great that you cannot hear Radio Fly
Speck at all.
This impedance mismatch is akin to placing an attenuator in-line with your
feedline.
In which case the tuner will "tune out" the loses caused by the mismatch and
allow Radio
Fly Speck to rise above the receiver's noise level. Including the static
crashes to of course.
But there you are - Fly Speck Radio becomes audible. Works for me!