SuperAdmin have access to all subsites in the network.
When Membership is active on subsite (not network active), SuperAdmin can access this page but after he click on blue "test memberships" link in the admin bar he is locked out of this subsite with the message:
You attempted to access the "SUBSITE NAME" dashboard, but you do not currently have privileges on this site. If you believe you should be able to access the "SUBSITE NAME" dashboard, please contact your network administrator.
During the chat we confirm this issue and we find a workaround
But this is a bug which should be fixed.