Is it true that PGP has uses that do not overlap with Signal? If no, then please ignore the rest of this comment.
Is it true that a block cipher has uses that do not overlap with a stream cipher?
Signing and verfiying files.1
Encrypting files.
From what I have read, Signal is concerned with messaging. But there is more to PGP than just messaging. Files that need to be signed/verified or encrypted do not necessarily always have to be sent to or received from others.
The blog post singles out one particular use of PGP, i.e. email, and complains about one implementation of PGP: GPG. Maybe the GPG software is not user friendly. Maybe PGP email is hopelessly complex. It is easy to make those arguments. Many have done it before.
However PGP encryption, in the general sense, not when used for email and not necessarily as implemented by the GPG software, can still be useful.
Files can still be useful, even when not in the context of the "messaging" business, Twitter subsidiaries, Facebook subsidiaries, advertising, smartphones, etc.
1 I prefer ed25519 for signing and verifying files but perhaps there are advantages to PGP I am not aware of.