Doesn't this assume you have the "spec" before you start implementing it? But then how do you estimate how long it will take to come up with that spec?
If you have a very good detailed spec you have already done much of to work to make the implementation easy.
If the spec is high-level and "fuzzy" it leaves the work of "resolving the spec" to the programmer.
So trying to estimate the time it takes to code a system depends on the quality of the spec, and therefore is difficult if there is no standard on how detailed the spec is to be.
If you have a very good detailed spec you have already done much of to work to make the implementation easy.
If the spec is high-level and "fuzzy" it leaves the work of "resolving the spec" to the programmer.
So trying to estimate the time it takes to code a system depends on the quality of the spec, and therefore is difficult if there is no standard on how detailed the spec is to be.