I can’t speak on behalf of Techlore–and I would also be interested to hear what factors led to the selection of Discourse and what other platforms were considered–but the reason I personally would probably choose Discourse or another modern take on forum software (beyond the fact that it is open source) is it feels like it has a lot of quality of life and usability improvements and features, that make it a lot nicer to use, compared to the older forum software from the 90s and Y2K era.
Also consider that: Tor Project, Whonix, Qubes, GrapheneOS all use Discourse for their community forums as well. In my opinion, Techlore is much much further down on the spectrum of severity with respect to the average member’s threat model and risk compared to these other communities. If it is secure enough for them, it seems like it should be reasonably considered secure enough for us.
Are you aware of any other forum software that doesn’t rely on JS and feels both modern and featureful and is also open source?
edit: FWIW, I’m sympathetic to your concerns, and I am one of the tiny “handful of folks” that @anon52464727 alluded to that blocks most JS by default (actually only 3rd party JS which is a lot less extreme than blocking JS outright… but still–on the spectrum)