To be clear, per Amazon's answer, they brought "content threatening violence" to Parler's attention in November. It would be a stretch to characterize anything I see in Amazon's filings as "planning" an attack.
And it's possible that even in the week leading up to January 6th the absolute number of violence-inciting, electoral-count-related posts on Facebook, or in YouTube comments, was higher than on Parler, just because those platforms are so much bigger. But on Parler, it seemed like the actual majority of comment traffic was either "stop the steal" complaints (with a range all the way from "here's another interesting affidavit" to "bullet to the head"), or memes mocking the appearance and assumed sexual history of famous Democratic politicians including Obama, Pelosi, and AOC, or anti-vaxx/anti-mask attitudes toward the pandemic. I saw all of that on Facebook/YouTube as well, but on Parler there was little else... at least in the comments on the posts of any "influencers" I checked.
I would guess actual "planning" also took place using Discord groups, Telegram/Signal, CB radio, phone calls, regular email, and like-mided people literally meeting in the street and chatting.
And the HORC isn't saying that the FBI shouldn't look at at Facebook and Google and other communication tools; they're just stating the obvious, that Parler is a good place to start and a place the investigators shouldn't ignore.
(By the way, the committee acronym HORC reminds me of the Spanish noun horca, a hangman's gallows. Weird coincidence?)
I went on Parler myself, posted something pro-vaxx and a link to the National Archives, and earned the threat "you better watch your 6 commie, we're coming for you". It was really bad.