After my second email I got this response:
Hey,
Thanks for contacting us. We're currently looking into the reason why these cookies are set and checking to see if we can remove them without any repercussions. I'll let you know as soon as we've got an answer for you!
Best,
Phil WordPress.com | Automattic Inc.
I'll update this if they contact me again.
EDIT: I haven't got another email from them, but it looks like those cookies no longer are set in Gravatar image requests. Thanks Gravatar!