So only documents released under a Freedom of Information Act (FOIA) request need to carry the justification or authority for why the information was redacted. Documents with classified information that the government chooses to release to a general public do not need such authorities or justifications for redaction of information.
It's also important to know that two or more pieces of Unclassified (U) information may raise the information to a level where the information can be classified and a lot of this document looks like it's combining more than a few sources. In addition, while employment at an Intelligence Agency is not something that is classified in and of itself, it is still protected information for people who aren't on a political stage within the agency. Jimmy Jones (1), an FBI computer analyst (2), may be named on that list somewhere because of work Mr. Jones did, presumably analyzing computers, did find some information that aided in the investigation (3).
Alone, none of those three facts may be classified, but together, they can name an individual who can be approached to gather information about the case from... either by a reporter or someone worse. So for that reason, Mr. Jones is hidden on the list.
Other redaction could protect sources and methods used to gather the information and in one case, I saw the redaction of a number and a list of names that were being investigated at present time. Typically you don't let people know they are under investigation, so those leave.
Since this is an aggregate document from multiple agencies, it's the individual agencies job to "sanitize" the information before it is delivered to Congress. Ultimately Congress, being the source agency of this document, is charged with sanitizing for the public release, but they must respect all agency classification decisions and remove any source material that the agency said they could not release.
Edits to answer concerns by OP:
So yes, it appears that these rules only apply if the document is released in relation to responding to a FOIA. If a government authority chooses to release a document on its own without fulfilling a FOIA Request, then they are not obligated to the justification or authorities.
That is correct. Additionally, as the investigation is currently ongoing, it may mask identities that are either informing on a criminal action or are under a criminal investigation. There may also be people who are "incidental". Suppose there is a wire tap on B.A. D'Gai, who calls up his dear sweet Granny D'Gai to let her know that he will be in D.C. on a certain date. That particular date range shows evidence that can establish D'Gai was or was not in town for a crime, but we don't need to know it was Granny D'Gai who he gave the date too. Basically the Jimmy Jones example is one of many reasons why you would leave someone's name out of document.
There are one of three reasons for this that these (U) markings are appearing. The fist is aggregate data... the table may be unclassified, but there could be information elsewhere in the document from another source that would put the classification higher up when combined with the table. The second reason is that since this document was likely written by a representative, the proper handling procedures might have slipped his mind and he put down (U) either as a mistake OR under the assumption that nothing would be redacted prior to an intended public release. Despite him marking everything as (U), this does not downgrade the classified nature of the redacted information. Third is that the document was in fact classified to the proper level and the table with the unredacted information had a higher marking. However, during sanitatization, a new document was created which downgraded everything that is unredacted to unclassified and everything was re-marked with the (U). As you mentioned, the full document does have a mark on the bottom of each page saying that it was originally very highly classified. (The other unredacted former marking NOFORN means that the information isn't even allowed to be released to our most trusted allies because of sensitivity concerns.)
