There are plenty of evidence of Hasan Piker's antisemitism to the point that people have been collecting and contextualizing them:
https://whoishasanpiker.com/antisemitismAlso plenty of groups fighting against racism and antisemitism have come out against him (ADL, AJC), it's easy to Google.
That House resolution is just the latest good indication that it's not just some isolated incidents.
Piker's only answers are that he's taken "out of context" or that he's "joking", there's literally no context that make his public statements better or less antisemitic though, if you have any I'm interested. He also publicly voice his support to groups that are openly antisemitic (Hamas and Hezbollah) and recognized as FTOs (Foreign Terrorist Organization) by the US government.
Now, it's his First Amendment right to be this way but Twitch/Amazon share a responsibility when they platform this content, and even more when they apply moderation selectively, in my opinion that should forfeit their Section 230 protection. They are deliberately shaping the discourse on their platform by permanently banning other streamers with opposing views and never taking similar actions against Piker.
"Fun" fact, Twitch even had to carve out their own rules because Hasan Piker couldn't stop himself from calling Jews in general or some groups of Jews "zionist pig-dogs" and "inbred": https://www.nbcnews.com/tech/twitch-changes-hateful-content-...
EDIT: Regarding the bills of attainder, it likely doesn't apply here because they aren't trying to punish these two antisemites specifically, they just use them as examples of the proliferation of antisemitic content on platforms like Twitch and want Congress to condemn this practice and encourage platforms to address the issue.