> Are they admitting that their app sandboxing is so weak that a malicious app can exfil data from other unaffiliated apps?

An app can read the content of notifications if the appropriate permissions are granted, which includes 2FA codes sent by SMS or email. That those are bad ways to provide 2FA codes is its own issue.

I want that permission to exist. I use KDE Connect to display notifications on my laptop, for example. Despite the name, it's not just for KDE or Linux - there are Windows and Mac versions too.