In the Document Restoring a Project from backup below is mentioned.

Note, the Project Import tool will lock out your instance of JIRA during the actual data import (not during the validations), so please ensure that your instance does not need to be accessible during this time. 

I have tested the same using JIRA version 7.7 and I was able to access JIRA. Either please remove this point from the documentation OR if JIRA applies a lock in the backend some where then it will be helpful if there is a better explanation.