Deakin, your itinerary qualifies, as confirmed by Thai airways.
Call centre staff are not always well versed in visa requirements, and referring you to the embassy puts the onus on you, not them (which is perhaps understandable as ultimately it is your responsibility). Airport staff are in the best position when it comes to these issues and your itinerary is very straight forward, with no complications (such as a return to origin with a short transit stop).
Thai advising you to check again before travel is good advice. TWOV rules recently changed in Guangzhou, with no notice.
Bottom line... you're good to go, with a very easy itinerary that is clearly within the rules.