They were definitely extreme by the standards of the time. Their aim was explicitly to completely revolutionise European politics, culture, religion.... everything. One comment I heard recent (on The Rest is History podcast, I think Tom Holland said it) they were the most radical movement in European history.

Their ideology implied at the very least getting rid of whole populations. They wanted to reset to an imagined ancient culture and rewrote history to justify it. Mostly imagined, anyway - Sparta was the one real example they looked to.

What you're actually asking is whether people knew they were extreme. But this makes your overall point circular: we can't say a party is extreme if the majority of people don't call it screens.