Sorry your birthday ride was cut short but thankfully you weren't hurt!
Considering the failure, you've now got me wondering about my new bike (see photos) that also has a sloping top tube and a long-ish seat post. I did buy an alloy (not carbon) post, but still, I worry about the loads on the carbon seat tube.
I weigh in at 157 so not quite as heavy as you, so that's in my favor, I suppose. Hopefully this isn't a major design flaw with this type of frame design though.
I've been riding the bike for about a year now (almost 4,000 miles on it so far) and it's still good . . . but I will pay more attention to the seat tube area now!
Rick / OCRR