That article by JoeDirt tries to conflate destroying evidence with withholding information, which are different things. The allegation of destroying evidence appears to be one guy's (Loudmilk) statement that he can't find stuff he thinks should be there. So when his staff eventually finds the stuff he wants to look at, he can say "Found it. My bad." and go back to whatever he was doing. Or, as you noted, they can go back to the source (or a half dozen other Jan 6th document repositories) and get the information there. That may be quicker if, as Milk also says, he believes the Jan 6th Committee didn't log and index all the information properly.
"Withholding information" is different and seems to be a catchy conspiratorial way of saying certain information wasn't cited in the Jan 6th Report. Not everything gets cited in a Report. That is true for every Report ever created. Information that authors don't consider relevant or pertinent doesn't get cited. But anyone else is free to cite it for whatever reason they want -- i.e., anyone from a US Senator to a journalist to an internet fool can write their own Report citing information they like, if they want.
Also, the "stunning" Steve Sund stuff (SSSS) is all cited in the January 6th Report. There are dozens of citations to his testimony (both February 23, 2021 and April 20, 2022 interviews) regarding preparations for the January 6th Trump rally. See Appendix 1 ("GOVERNMENT AGENCY PREPARATION FOR AND RESPONSE TO JANUARY 6TH") and Appendix 2 ("DC NATIONAL GUARD PREPARATION FOR AND RESPONSE TO JANUARY 6TH").
