It really just means they believe the allegations are untrue and that they think the evidence will demonstrate they're untrue or there won't be enough evidence

It really just means they believe the allegations are untrue and that they think the evidence will demonstrate they're untrue or there won't be enough evidence to prove them. The reason this is often stated or quoted is that a defendant has to make a statement to this effect when replying to the complaint. However, at the time of the initial reply, no discovery will have occurred and therefore no evidence will be officially available to either prove or disprove the allegations. Accordingly, all you can really say at that time is "we disagree and believe the allegations in the complaint are not true and/or can't be proven."