Bah, it's a perpetuated myth. I've used tanks that had been used for copper treatments many times to house inverts. Never a problem. I mean, where do people think the copper is lurking? Glass won't absorb it, neither will silicone. Even if the latter did, it's going to be such a tiny amount as to be negligible.
You certainly don't want to be using rocks that had been in copper, but the tank is fine.