99% of the time a speaker is described as a transmission line, it means that the bass augmentation coming from the rearward radiation of the speaker is dominated by 1/4 wavelength behavior. In reality, the effective length of the line and the characteristics of that augmentation can be altered by dampening, as well as tapering or widening of the line, or the insertion of a port.
Bass reflex, back loaded horn and 1/4 wavelength transmission lines can be thought of as three points on a continuum, with every speaker having openings in the box exhibiting some degree of behavior from each part. Tall tower speakers, for example, are sometimes modeled as bass reflex, but end up behaving like mass loaded transmission lines.
The term 'transmission line' to describe a long channel for absorbing the backwave of a driver is technically correct, but rarely used to convey this meaning. Few speakers attempt these types of transmission lines; the Linkwitz LXMini and the Nautilus are good examples. Fried, IMF, PMC and so on all refer to their 1/4 wavelength speakers as transmission lines, and I believe DCM did with the Time Windows, if we're being really esoteric.