There is a subtlety to what you wrote, but in practice that is right. It is not actually the master bus which clips, but the point that the output of the master bus is converted to fixed point to send to the audio interface, or to a fixed point file format such as wav.
The only reason I make the distinction is that means all you need to do is reduce the output level with the master bus fader and the audio will not be clipped (assuming you do not have any plugins in the chain which clip internally).
Internally Ardour by default also writes float32 wav files when recording (although this can be configured in Session > Properties > Media).
During export, yes, if a fixed-point sample-format is used, it can clip - and the export analysis will show this. This is why normalization during export comes in handy. A gain factor can be automatically applied to prevent clipping. see The Ardour Manual - Export Format Profiles
Good point, I guess to be fully correct I should have noted PCM (i.e. specifically fixed point) wav files.
I think Ardour will let you export floating point wav files, although that is primarily useful for importing into another DAW, most playback devices need PCM files.