It can't hurt to try but I think it has to be in person if it's (a) your first time, or (b) your first time using a new passport.
Given that we now know it's not integrated with the main immigration system, I'm guessing your first visit is where they capture your passport details/image(s) and put them in the TM47 system.
I'm guessing the person who reviewed it looked in the Immigration system. The really polite and informative message suggested it was not computer generated 😁
The 23rd Aug was the 90 days after my previous exit/entry (a trip to Pnomh Penh). When I reentered at Mae Sai, the Immigration system stored 28 Jun as my arrival date, so my new reporting date is September.
The TM47 system has no record of that arrival, as it doesn't pull your arrival date from the Immigration system.