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).
Trello Enterprise (optionally) would secure your content (i.e. attachment restrictions, power-up restrictions, token restrictions, audit logs, team management).