Standard disclaimer: IANAL, as such this is not legal advice.
The case is actually not clear cut. In my opinion there are two different works here: the design of the template, and the code behind the design of the template. The code does not actually make it into the final output PDF, so the GPL is not actually relevant. The template is what makes it less clear cut. It may very well be that such a template is not creative/original enough to be copyrightable.
We can actually look at another program similar to LaTeX. Lilypond is a music typesetting problem, and its license is GPLv3 (plus a font exception for PDF embedding). Probably no sensible person would argue that the default style of lilypond output requires the output to be GPLv3.
There is a valid question: what is the difference between \documentclass or \usepackage and an import statement in python, that would cause the latter to require GPL compliance? The difference is simple: import introduces a runtime dependency. No dependency is introduced when \documentclass is used.
UPDATE: I see that a lot of the arguments made relies on the architecture of LaTeX. I don't see how this matters - if you use work A to produce work B, whether B is a derivative work of A is a property of B itself, not the process used to produce B. The reason that architecture matters for GPL vs LGPL is that static linking vs dynamic linking produce different works.