I don't think that we can say labour is exploited in the sense Marx intends
because it is the source of all wealth (which as you point out, and which
is obvious, it is not). Labour is exploited, of course, in the sense that
nature is also, namely, that an advantage is derived from some use of it.
In the sense Marx intends, exploitation is the socially produced and
maintained compulsion of a labouring class to work for the benefit of a
propertied class in addition to any work done directly or indirectly for
its own benefit. Obviously, nature cannot be exploited in this way.