According to the same paragraph, it's because Signal isn't a "browser app":

> Given that Windows doesn’t let non-browser apps granularly disable Recall, Signal cleverly uses the DRM flag on their app to disable all screenshots.

(emphasis mine)

Apparently, Microsoft consider browsers special:

> While it’s heartening that Microsoft recognizes that Web browsers are especially privacy-sensitive applications, we hope they offer the same granular ability to turn off Recall to all privacy-minded application developers.