It would be better for the company to quote an annual number...

It would be better for the company to quote an annual number than this monthly number. Monthly churn is misleading and doesn't give the average investor a decent sense at a quick glance of what churn looks like and how risky the business is.

A sense check that backs up the 12% annual churn rate is the life of a subscriber shown of ~8.5yrs which implies annual churn of 12%.

I think you are being a bit pedantic on the maths. Technically the times 12 is not perfect maths wise but it is going to be pretty close for small numbers at 1-3% monthly churn. It is highly likely that annual churn is somewhere in range 11-13%.

The approximation of x12 wouldn't work as well for larger numbers above 3% (and definitely doesn't work for 50% monthly churn). But even for monthly churn of up to say 5% it would give a better finger in the air estimate than the lack of disclosure from the company on annual churn.