Don't believe Apple Carplay has a calendar functionality per se, though I think it's the potential to access similar calendar app (on your iPhone) through Carplay.
But the key point for the TD / regulations is that Apple Carplay also has zero bearing on the driverability of the vehicle, the premise where Tesla calendar hasn't been approved (or Tesla HK doesn't even bother to apply to the TD or contest TD's formal / informal decision).
The question then is why is Carplay approved but not the Tesla calendar?
About the calendar, I think TD's response was pretty clear that the calendar app does not meet legal regulations (they used wording in a roundabout way though). The only objection they took was to Tesla's specific wording in their software update ("no bearing on drivability"). They never suggested there was any way for exemption.
As for Carplay, it may simply be a case of selective enforcement. TD already had a target on Tesla's back from the whole autopilot release. The public embarrassment of TD over the calendar issue only made things worse.
From Apple's site they describe the following for CarPlay:
"get directions, make calls, send and receive messages, and listen to music"
Other than perhaps the messages, I'm pretty sure almost all infotainment systems support those functions. I don't see a calendar app in the default apps (there is phone, music, maps, messages, now playing, podcasts, audiobooks).
iOS - CarPlay
As for user apps through Carplay, TD may not necessarily have scrutinized that as closely. Technically the app would be running on the user's phone, and TD may be unable to enforce that (I'm pretty sure people in Hong Kong also have those phone holders to put their phone on the dash). Also the app would not be installed while doing type approval. This is different from Tesla's case, where the app is running on the screen itself, and the app is a Tesla app installed by the car manufacturer.
Maybe if the public complained to TD with a concrete example of a calendar app (or some other app not included in the default categories) running on a Carplay vehicle, then something might happen. I suspect what might happen however is TD simply asks Apple to restrict that app so it is not allowed in the Hong Kong region.