Evolutionary theory cannot be so easily demonstrated.
True, but neither can atomic theory, yet we don't have folks running around denouncing it on nearly every science thread or forming local chapters of the "Anti-Atomic League".
Nor does that invalidate the statement that evolution is "as valid or demonstrable as the fact of gravity" -- it is. Both can be validated and demonstrated to anyone who cares to pay attention for a bit with an open mind. Likewise for atomic theory, and so on.
For that matter, gravity isn't all that trivial to demonstrate either. Sure, you can show that "things fall down", but that doesn't necessarily prove *gravity* (remember the old joke, "there's no gravity, the Earth sucks"). Establishing *gravity* is not so simple. Recall that it took Isaac Newton to make the realization that a universal force proportional to mass and inversely proportional to the square of the distance was at work on both falling apples, *and* celestial bodies, and that it took Galileo to realize that contrary to intuition (and discounting air resistance) heavy objects fell just as fast as lighter ones (and why). Parabolic ballistic arcs are a direct result of gravity, and yet the great Aristotle believed that a thrown object travels in a straight line to the top of its trajectory, then loses all momentum and plummets straight down.
Actually demonstrating gravity, its properties, its universality, and its consequences is not something that can be done in a few minutes, or is immediately obvious from day-to-day experiences with falling objects (or falling people). And yet it *can* be validted and demonstrated given enough time. And so can evolution.
For that matter, the driving forces of evolution are *far* better understood than the root cause of gravity. Physicists are still pretty much 100% in the dark about whence gravity springs, or how it exerts its force, or how fast it is propagated, etc.