I remember seeing that on his avatar since the early days of the WP Engine drama.

It left a temporary spot in my vision after I scrolled it away.

What does that mean? Why is it a surprise?

It’s surprising that Twitter even supports that.

Some old twitter users still have animated gif avatars that stop being supported years ago. Maybe their approach is to just stop supporting it for new avatars and grandfathering whoever manage to upload one before.