The info you've been given is correct in that all commercial CDs confirming to the "red book" standard have two audio streams, intended to be used as the left and right channels of stereo recordings. That said, there are many examples of recordings original made for mono reproduction (which includes just about every "popular" recording from the invention of the gramophone to the mid 1060s) that are now available on CD. The mono version of the Beatles' box set is the best known example, but there are many others. In these cases, the data content of the two audio streams on the CD is identical, so even though the representation is "stereo" the sound will be strictly monophonic. The main reason that these exist stems from the period between the introduction of the stereo LPs and corresponding "hi-fi" equipment and these becoming the dominant format - over that period many records were released in both mono and stereo versions, and in most cases the former wasn't created by mixing down the stereo version. Mono and stereo versions were often different mixes or even different performances - hence the appeal to fans and collectors of having both versions available in modern formats. Of course, before the introduction of multi-track recordings the "stereo" versions were often a simple split from the master recording, for example with guitars and vocals all in the left channel, drums, bass and piano all in the left.
Just to make things more complicated, there was a short-lived trend to "create" stereo LPs from mono recordings by splitting the original into "left" and "right" channels and using effects such as reverb or phase reversal to make the channels different - generally with pretty gruesome effects, much like early colorization of black and white movies for color TVs. Unfortunately, these "electronically processed for stereo" recordings sometimes became the standard masters for LP pressing and some were even used for early CD releases.
Bottom line is that although all CDs have two audio tracks they can be identical resulting in mono reproduction; AFAIR there is / was an alternate standard developed in the early days of compact discs that allowed for genuine mono (one channel) reproduction, mainly intended for spoken word recordings, and potentially allowing up to 148 minutes of audio on one disc (double the 74 minutes of the original CD spec). Obviously these never took off commercially, and were superceded by other digital formats.