Here’s another study from Feb 2020:
Reduction and Functional Exhaustion of T Cells in Patients with COVID-19

In short: In some cases (unknown exactly who or why) SARS-2 can directly infect the T-cells. This results in a milder set of symptoms, but serious immune system debilitation. This leaves people vulnerable to other opportunistic infections.

One marker from other studies I’ve seen: A second Covid infection is somehow much worse —for the immune system— than a first case.

And yes, one possibility is SARS-2 infecting T-cells could be one of the causes of long-Covid. There seem to be several different causes of long-Covid symptoms, it’s still unclear wtf is going on. Probably multiple things, IMHO.

Don’t catch Covid. But if you’ve had it once, definitely don’t catch it again! That seems to be a good rule of thumb so far.