I'm always sceptical about GDPR being that useful, as surely they only have to show information specifically logged against you, not data in another database that could theoretically be traced back to you but is not directly linked on their system.
I particularly love how they say your IP address is not logged from their addon but the data is linked back to your speed tests, which include your IP address. So were getting into semantics there as it suggests the sites you visit CAN be easily traced back to you, quite directly if you have GPS enabled on your phone.