My reading is that they are not asking "give us your disinformation" they are asking for their policies and data regarding the moderation of disinformation. Their response to the request seems to more or less answer the request by saying "we don't have any because we consciously don't try". This request appears to also be just a request. Nor a subpoena. But that being said, Congress isn't law enforcement. They're allowed issue subpoenas for information relevant to informing legislation. Mark Zuckerberg wasn't under indictment, nor was a material witness when he testified to Congress. They just wanted a public forum to understand how social media functions, how it could function and if additional oversight was warranted.