They are orthogonal.

Constructive: you need to generate an object satisfying some particular properties, this requires non-trivial thinking.

Implementation: the solution is relatively easy to come up with, but difficult to implement (e.g. the code is long, bugs can be introduced easily, there are many possible off-by-one errors, etc.).

You can think of the 'constructive' tag as a measure of how much ad-hoc thinking is necessary, and of 'implementation' tag as a measure of how much programming and debugging skills are required.

first one takes either 0 or 10 milion years to think and 0 seconda to code and the other one takes 0 seconds to think and 10 milion years to code