Online index operation blocking inserts
Rebuilding an index online doesn't allow you to completely avoid locking - when the process completes the table is locked for a brief period and the new index replaces the old one. 

Online index operations need to take two very short-term table locks. Shared)table lock at the start of the operation to force all write plans that could touch the index to recompile, and a SCH-M (Schema-Modification) table lock at the end of operation to force all read and write plans that could touch the index to recompile.

And the article you provided has some info on this as well: