If you‘re opening the cover, disconnecting antennas might be the way to go instead. Depending on the device, it‘s relatively painless and even reversible.
I believe they mean a cover as in a case that has a folding cover, not as in the external layer of the phone itself. So you effectively turn that otterbox-ish thing into a faraday cage that will enclose your phone.
I am not sure how I follow how that isn't completely negated as soon as you go to actually use the phone, though.
Most antennas require desoldering to disable, but mmWave & possibly NFC can be removed without it. Lining the case thoroughly with EMI tape edge-to-edge and adding another layer over any gaps blocks RF signal emission & reception. The screen side doesn't need shielding since antennas aren't directed through it. If you set & use USB-C for charging only and your installation passed PGP & attestation, then the OS disabling wireless components will also eliminate active remote attack surface & reduce passive remote attack surface at least remarkably.
I am not sure how I follow how that isn't completely negated as soon as you go to actually use the phone, though.