what’s stopping me from just screenshoting
Images of text? Accessibility.
So, another sacrifice at the altar of social media to deliberately break a straightforward feature at the expense of people who need accessibility.
Social media can be such trash.
Passkeys or WebAuthn are an open web standard, and the implementation is flexible. An authenticator can be implemented in software, with a hardware system integrated into the client device, or off-device.
Exportability/portability of the passkey is up to the authenticator. Bitwarden already exports them, and other authenticators likely do, too.
WebAuthn relying parties (ie, web applications) make trust decisions by specifying characteristics of eligible authenticators & authentication responses & by checking data reported in the responses. Those decisions are left to the relying party's discretion. I could imagine locked-down workplace environments allowing only company-approved configurations connect to internal systems.
WebAuthn has no bearing on whether an app runs on a custom platform: that's entirely on the developer & platform capabilities to reveal customization.
Possible, unlikely, worse. The most accessible content is native text with the full content structure of the original. Quoting without compromises & feeble games would be better.