That is such a stupid argument I don't even know where to begin, so I'll just list a few things off.
1) how do you word the pardon so that it will be legally defensible if the DOJ tries to bring charges against someone? Who do you name as the persons to which the pardon applies? Tell me your verbiage and I'll poke a thousand holes in it.
2) are you aware that the DOJ has an entire office that handles the pardon process, and that pardons can take up to 5 years to process?
3) are you aware that every memo, every order, that comes out of the WH undergoes review? In this case, the WH Office of Legal Counsel would review it. Guess where they're located? They're an arm of the DOJ
4) Yes, that same DOJ that had just spent 4 years targeting and trying to remove Trump. You think they won't stonewall for 14 days at the end of his term?
5) which also brings up the point that most of the federal government wasn't taking orders from Trump after 1/6. WTF was he gonna do, fire them? It takes weeks to months to fire federal employees.
6) the federal government can't do anything in the span of 2 weeks. And especially if it's something they don't want to do. Anything that Trump tried to set in motion, Biden would have reversed or cancelled on Day One.
7) And again, how are you yo gonna word that pardon? Who does it apply to? Everyone present at the capitol that day? Does it apply to people who actually did assault police officers, or just the sightseers? How do you phrase that? You can't do it in a way that is legally applicable to only the people you're wanting to pardon.
8) that problem is made worse by the fact that the large majority of people were arrested and charged after Trump was gone. Stop and think about that and how it affects your wording.
No pardon was possible within that time span. It's always been a dumb argument.