Although I don't own an Apple watch, they more than likely do have a specific serial number, possibly an IMEI, and can tell the type of data that comes through their network. Most carriers screen their traffic to determine how and what each person is using on their network. Whether it's a phone, tablet, some form of tethering, etc. It is actually fairly easy for Telus to block things they don't want you using, especially if you are using something your plan wasn't intended for.
Don't get me wrong though. I do believe a person should be able to use their data for whatever they want. if that is tethering a desktop PC, a tablet, smart watch, setup a mobile hotspot, or whatever else. You pay for it and should be able to use it how ever you want. You could complaint to Telus about the practice, which probably won't get you anything other than the frustration of waiting hours on hold to get hung up on. Perhaps consider filing a complaint with the CRTC and hope they aren't feeling too corporate sympathetic when they finally get around to your complaint.
Unfortunately, there is a disclaimer in the article indicating the add-on is for shared plans only:
"Carrier update for Telus iPhone enables Apple Watch data support $20 activation fee $10 a month includes one gigabyte of data to add to a shared data plan"
For instance, those folks who last year jumped on the 10 GB for $60 plan, find it is not eligible for either the Apple Watch add on, nor device upgrades.