Sensibly this is the most reasonable explanation but it is still incomplete. The Cant can be understood by other streetwise people, not only thiefs. The Thief however has a different take on the common slang. His Cant in indecernable from the local dialect slang, to all except another thief. The order and inflection of words make a complex language hidden inside a dialect wrapped in slang so to speak. Slang and dialects are of course highly localised. So each area or city has its own Cant version. The Cant is set by the guilds who themselves have their own subset, and this changes frequently for security purposes.

An example to illustrate this would look thus, a thief from Bissel visiting Greyhawk would not understand Cant and much of the slang, but would know enough to identify himself as a thief to another. He would not however be able to readily identify another thief, but must trust his instincts. His approach however would be unnoticed by an innocent target

Within the same city or area thiefs could communicate readily enough. However take three thiefs, two from the MoonShadow Guild the other from the Cresent Moon Guild. All could converse business woth each other in a tavern but the MoonShadow Thiefs could also talk to each other excluding the third. The third theif would understand the subject of the conversation in a very general way, but not what was being siad. He would for instance know that they were talking about time and present company, but would not understand that one is asking the other how long the poisen injested by the third thief takes to kill him.

DMs should create general and rough guide lines, not to draconian, on how distance effects the ability of the theif to speak Cant. Note: the Cant is an excellent way for guilds to identify strangers on their patch and protect their interests. If your not in the Guild your out of the picture.