A small number of accounts get Pro for free as a thank-you — most commonly beta testers, whose Pro access is tied to the beta program, or occasionally a permanent complimentary grant as a reward. Either way, the Plan card at Dashboard/Profile (avatar)/Profile…/Plan will say "You have free Pro access" instead of showing billing controls, since there’s no subscription behind it to manage.
If you’re not sure which applies to you, that same Plan card spells out which kind of complimentary access you have.