Universal Approximation - Are ReLUs discriminatory?
Yes, ReLUs are discriminatory. See for example the nice, simple proof in Lemma 3.15 of these notes by Leonardo Ferreira Guilhoto:
http://math.uchicago.edu/~may/REU2018/REUPapers/Guilhoto.pdf

Regarding whether an unbounded function like a ReLU can be discriminatory, note that you can construct a bounded, continuous function as the difference of two ReLUs:

$f(x) = ReLU(W x + b_1) - ReLU(W x + b_2)$

and an appropriate choice of $b_1, b_2$ makes this difference f a sigmoidal function, as defined in Cybenko, so function f is discriminatory. 

By definition a function is discriminatory if the zero measure is the only signed Borel measure $\mu$ such that:

$\int f(W x + b) d \mu (x) = 0$
for all W, b

Applying this to the difference of ReLUs above shows that both integrals over each ReLU must have measure zero if integral of f has measure zero.