No, you didn't read what I posted in reply to Mark originally.
myisam performance is not just down to the number of SELECTs you have, it's more complex than that and involves taking into account which of your queries will lock the entire table as they execute and which other queries may be running concurrently.
Any SELECTs involving joins, UPDATEs and INSERTs (in certain curcumstances) will cause the entire table to be write locked which may or may not be a problem depending on what else is going on at the same time.
For Marks tests, he seems to be doing quite intensive operations on a relatively large data set of 4.6m rows. I would hazard a guess that if he introduced some other query types on that data set to run concurrently (as one might get on a high transaction system) he would see some undesired locking situations.