For bank statement line having an account_id, but no journal_entry_id, it is not possible to reconcile the line in the bank statement reconciliation tool, as a filter is applied to only reconcile lines having journal_entry_id AND account_id not set.

As written in the help message of the account_id field:
This technical field can be used at the statement line creation/import time in order to avoid the reconciliation process on it later on. The statement line will simply create a counterpart on this account

Not allowing the reconciling should not prevent to close the statement in such a case. The button "close" was displayed only when all lines had journal_entry_id set.