It's the other way around. You only need to buy Atlassian Access to get the SSO + enforced 2FA for your Trello (and also any other Atlassian product) users.
Trello Enterprise (optionally) would secure your content (i.e. attachment restrictions, power-up restrictions, token restrictions, audit logs, team management).
I'm sorry it's confusing. It's trying to say that SAML SSO is provided via Atlassian Access - but you don't need to buy Enterprise to buy Atlassian Access - it's a totally separate product (and does not require Enterprise). We are in transition right now (formerly SSO was provided by Enterprise) and so our final pricing page isn't quite where it needs to be.
Trello Enterprise (optionally) would secure your content (i.e. attachment restrictions, power-up restrictions, token restrictions, audit logs, team management).