Do they? Let's check your source.
> To investigate this hypothesis, participants aged 20–34 perform a concentration and attention test in the presence and absence of a smartphone. The results of the conducted experiment imply that the mere presence of a smartphone results in lower cognitive performance, which supports the hypothesis of the smartphone presence using limited cognitive resources.
So, no. The presence of your smartphone on the desk in front of you is distracting, but that distraction goes away if you remove the smartphone. That's not "cognitive decline."
> Sorry, that's a narrative argument
No, that's me pointing out a competing plausible hypothesis. I'm not saying Covid is necessarily responsible for your anecdotal incidents; I'm saying that until you can prove Covid wasn't responsible, you have no standing to state conclusively that phones were.