Isn't an instrument solo only a single instrument being played? ?

As with just about everything in music there is no hard and fast rule. However, very broadly solo means "one instrument/voice is the leader or focus" (however this isn't a universal definition - a concerto features one instrument as a focus but you wouldn't say it's got a SOLO. Nor would you say that the singer in a pop song is the SOLOIST, even if they are the focus and only voice)

Sometimes "solo violin/voice/guitar/whatever" may mean "this is a piece of music written solely for one instrument" - i.e monophonic unaccompanied music.

Sometimes "solo violin/voice/guitar/whatever" may mean "this is a piece of music written primarily for one instrument, but is accompanied by others - such as the piano."

"Guitar/violin/whatever solo" usually means "a small section where this instrument is featured" and is almost always used where there are many other people are playing. Sometimes the solo is improvised, sometimes it is simply a section where the composer has written it just for one person e.g a soprano solo in a larger choral work.

No, in the context of rock and pop a guitar solo is simply the guitar playing over the top of the other instruments. Usually the other instruments play the same as the do for the verse or chorus of the song and the guitar plays something different. The same applies to sax or violin solo. The only exception is usually a drum solo where the other instruments stop playing.

A “guitar solo” isn't the same as “solo guitar”.

A solo when it comes to music isn't always linear. In that instance, the drums were use probably as enhancement for the guitar, as opposed to equal footing.

Drums only aid the Solo performer to keep the beat.