He actually compares the Tamron to several lenses, including the 500L. All he does is indicate what the difference in price gets you.
Here's a quote from it :
The Tamron zoom doesn't do too badly considering it's $4000 cheaper and about 1/3 the size and weight of the Canon prime. The difference would not be noticable in 4x6 prints and even in 8x10 prints it would be small.
This seems an eminently fair and reasonable statement.