INSERT... SELECT isolation?
No, MySQL builds a temporary result set from your query before performing the insert. 


  When selecting from and inserting into a table at the same time, MySQL creates a temporary table to hold the rows from the SELECT and then inserts those rows into the target table. 


Source: MySQL Documentation

Edit based on Comments

The insert is an atomic operation and so the data used to build the temporary result set will match the committed data in the source table at the precise moment that the query was executed (i.e. it is a snapshot of that point in time). 

After the temporary result set has been built any modifications to the source table will not be reflected in the results being inserted into the target table - unless you run the query again.