I ran the test with 1,000,000 INSERTs (taking 200 seconds), with the innodb cache size set to only 200,000 bytes. So there's a fair chance it updated the b-tree

I ran the test with 1,000,000 INSERTs (taking 200 seconds), with the innodb cache size set to only 200,000 bytes. So there's a fair chance it updated the b-trees on disk.