This seems to be a honeypot for associating your SSH public key with other identifying details.

It is a paid service, delivering a valuable developer tool, and which indeed uses ssh keys for authentication.

So, exactly what you said, but for the benefit of the user, and for the profit of the company, by offering an excellent product.

(I am a happy customer of their previous product, Sketch.dev.)

Hardly, you can use .ssh/config to configure an SSH key just for this service.