It explicitly doesn't. Read it again. Immediately after listing his 7 constructs he points out that a number of them are actually the same thing, and in the summary he clearly explains that there are only 4 different behaviors. His 7 constructs (which include methods and bare blocks) are actually only 3 or, arguably, 3.5/4 (his numbers in parens):
blocks, which aren't callable without being wrapped in a Proc in CRuby (1, 2)
Procs (two types: Proc.new/&block and lambda) (3, 4, 5, 6)
methods (7)
There two types of Procs, plain procs and lambda, with lambda's differences being how it checks arguments and its return behavior, as I noted yesterday in another comment in this discussion (http://news.ycombinator.com/item?id=1141030).
So, again, the original comment, "four goddamn callable types [Methods, Procs, Blocks, Lambdas]," neglects the fact that #call on the last three is actually just Proc#call.