Yes. Both the PO and TSA consider them flammable (they are designed to do that) and are therefore forbidden.
The fact that they are harder to ignite than the box you're shipping them in doesn't matter; it is a fact lost to them. Almost everything else around them would have to be already burning before the tablets would ignite spontaneously (just try lighting one in the wind).
It is virtually impossible to buy Esbit tablets along the trail, so you might consider putting them in your re-supply boxes and don't tell the PO about them. They don't have the time or capcity to check packages routinely.
When flying to a major city, I'll buy them when I get off the plane and before I go to the trail. I don't put them in my checked luggage (pack) on the off-chance that some other piece of my camping gear might trigger a curious TSA person to actually dig into my pack. I can't risk having it rejected. A box of 12 only adds 6 oz to my pack, and that will do me for about 3 weeks.