Secret evidence is certainly evidence in this case not least because it is absolutely extraordinary that the principal US  intelligence agency, let alone all of them, would make an issue about this for an incoming head of state, let alone anyone else. It just would not happen if this wasn't a serious problem. If Trump is not considered a compromised foreign agent, then we just have throw out that category altogether, for anyone at all, and accept that intelligence cannot say anything about the work that they do themselves.

It is far more reasonable to conclude that the CIA another spy agencies probably know quite a bit more than anyone else about how to do the work of recruiting and influencing foreign agents. So when they go through the extraordinary trouble of actually alerting competent political authorities, privately as well as publicly, that a political candidate, and subsequent elected winner, for the US Presidency (!) has likely also been a target of the same kind actions, to conclude anything other than that Trump is a foreign agent is simply being obtuse, on the part of those same political authorities as well as ourselves.  

At the very least it means that any geopolitical analysis that doesn't explicitly address the problem Trump's compromised relationship with Putin cannot be considered very solid.