I don't get the purpose, at least of his minimal example. The author says he wants to make his code position-independent, i.e., so that it can be executed from everywhere in memory (without relocation). But that is defeated by the...
They should have put labels in front of and after the string bytes, then most assemblers would evaluate "(labelafter - labelbefore)" to a constant integer giving the length as needed. No need for a runtime sub instruction either, then.
Yeah the example wont work, but since it's only used for getting the length of the string its an easy to fix to instead use pascal/counted strings with a length prefix byte.
mov edx, print
... in the example.