Some folks have sent in extensions as only tce, others have sent them in as only tcz. Though sending both formats has always been encouraged it has not been enforced. Since dealing with writable config files and such sometimes has to be done with a tcz, some less experienced extension makers tended towards sticking with tce's. Also, some folks have just sent in whichever is their preferred format.
Since we are moving towards tcz being the only extension type, and have support in TC 2.3 to use the tcz as either mounted or copy-to-ram, I do not frown on folks submitting extensions only in tcz format if they so desire.