I'm not sure your non-UK CAA has got it right.
FCL.015 is the regulation in question:
(c) A person shall not hold at any time more than one licence per category of aircraft issued in accordance with this Part
TMG isn't a category of aircraft. According to FCL.010 - Definitions, its:
a specific class of powered sailplane
I have both an ATPL(A) with TMG Class Rating & an SPL with TMG extension.