I had a project with 4 mono busses, which I exported as single mono files. Now when I import those wav files in a new session, they are imported as 2 channel audio files. Does this imply that my exported files were not mono really, or does Ardour import mono files per default as stereo tracks?
That is easy enough to check with any tool which displays the file info. On linux sndfile-info is the tool I would pick first, it displays sample rate, channel count, length, sample word size, etc. in an easy to read table.
Yes, if you create a stereo session, your master track will be two channels, and because tracks by default are routed to the master, they will be created with two outputs and a panner. It does not mean your track is stereo, just the output of the panner on the track.
That is not a stereo track, that is a mono track with a panner. That is traditional behavior of an analog mixer, a mono input channel will have a panner to position the apparent position of the mono source between the left and right speakers. Same thing in Ardour, although Ardour has enough flexibility you can change that behavior if you want by bypassing the track panner, or manually selecting the track routing.