Well, you're both wrong. You can't simply redefine the terms to win your argument and the term supercomputer doesn't necessarily refer to computer cores networked by a high speed interconnect. Come to think of it, the original post is absurd, because there is no way BOINC could run LINPACK which is the measure of the TOP500 rankings anyway. LINPACK stresses communication performance as well as scalar processor performance. BOINC would probably be slower than my desktop for that purpose.
Like a lot of these silly comparisons on slashdot ("My hammer is better than your screwdriver!") it comes down to using the right tool for the right job. If you're not in a hurry and your job is "a bag of jobs" type problem, use BOINC or some other distributed/cloud computing approach. If you're trying to solve some type of PDE use a purpose built system like Roadrunner. Using something like Roadrunner for the type of jobs that BOINC is good at is just a waste of resources - those networks aren't cheap.