...which is
a complicated matter to say the least...
It really isn't. A federal subcontract employee was injured on the job due to negligence by a federal employee, and sued the responsible party for damages. The federal government argued it held absolute immunity from civil suit, the case made its way to SCOTUS, and SCOTUS sided with the subcontractor and allowed the civil suit to progress.
Westfall v. Erwin, 484 US 292 (1988).
Congress shat its pants over this, realizing federal employees could then be sued
personally for acting like absolute fuckheads on the job. That brings us the Westfall act, which grants qualified immunity for federal employees while acting within the scope of their jobs, by requiring the federal government itself be named as defendant rather than the individual. The loophole to this, and Congress' convenient excuse for effectively negating a Supreme Court decision absent a Constitutional amendment, is sovereign immunity.
This
isn't "well just anybody could sue federal employees over anything" territory. That's called frivolity and it can be grounds for dismissal with prejudice, compensatory and punitive damages assigned to plaintiffs, and in some cases contempt charges.
...could suggest that it would use the same arguments to defend Trump for charges brought against him for the Capitol Riot, which is an assumption that goes well beyond the scope of the DoJ's decision in that case.
Okay, first, there are no "charges" being brought against Trump. He has been named defendant in civil suits. In this the form of the case, and the salient legal questions, are identical and therefore so will be the defense. A defense the DoJ is obliged to perform, because...
...Its position on the Carroll case hinges on the fact that Trump was a politician denying (albeit very crassly and inappropriately) public allegations about him...
...for one thing, Biden
also had credible sexual assault allegations levied against him which have been and will continue to be forcefully denied, and his own DoJ which I'll remind you is a department of the executive branch and therefore serves at the President's will, isn't going to lay the legal framework allowing him to be sued.
And this goes back to my exact point in the thread I linked: Biden's DoJ won't be doing a damn thing other than defend Trump because it is simply irrational for them to do otherwise. Any action taken
now will impact
their future potential liability in the event of scandal or upon leaving office, and Biden's DoJ is not going to lay legal framework that could be used against them in the future, let alone legal framework that erodes the unitary executive nor imposes any constraint on presidential power, privilege, and immunity.
Just the same as the Obama admin and Obama DoJ refusing to investigate Bush or members of his administration.
...Hell, the article even ends noting that there's a substantive legal distinction between the comments being defended in the Carroll case and those of the Capitol Riot, noting that while the comments about Carroll's allegations are at least arguably covered, those of the Capitol Riot almost certainly are not.
There is no substantive legal distinction, that's sound and fury, signifying nothing. The Capitol riot suits cite the Third Enforcement Act as their legal grounds for proceeding, and the Third Enforcement Act only applies to the states (and DC), not the federal government.