Yes you are correct. Post-menopausal bleeding is abnormal or unscheduled bleeding. A withdrawal bleed on sequential HRT is normal and is not classed as PMB. Having said that if it is suddenly very heavy or clotty etc then there could be cause for concern - so it may be abnormal, but not PMB as such
PMB is normally reserved for post-menopausal women not on HRT who suddenly bleed, or women stable on conti HRT for some while and then suddenly bleed, or possibly also women on sequi HRT who bleed at completely the wrong time.
The other important point is that the endometrial thickness will of course vary throughout the cycle, on sequential HRT and therefore the best time to have a routine scan that you are having, is immediately following the withdrawal bleed (even if in the very last stages of light spotting at the end), when it is at its thinnest. Only then can you tell if it is thickened or not.
So the most important thing to note is when you had the scan in relation to your bleed and how thick the lining was. Whoever wrote that you had PMB is wrong and has not understood what regime you are on. Any menopausal clinic should know this and take full notes on what your regime is.
In the circumstances and if you don't know at what point in the cycle the scan was done, nor the thickness (or even if you do know the thickness) I would get back to the clinic or GP and ask for it to be redone but make sure it is timed as above - and explain this to them if they don't know!
All the best
Hurdity x