Does that somehow mean they should be less skeptical and not require more evidence before publishing the whistleblower assertions as fact? Do whistleblowers, by asserting that they are whistleblowers even without evidence, deserve more trust in this scenario for some reason?

I agree with you, now its normal practice to blame someone without evidence. Intelligence communities around the world are major source of "fake news" and media manipulate. My understanding is that "whistleblowers" are part of experiments done by intelligence communities to gauge public response on certain activities(might be illegal ,immoral) and with passage of time we forget and accept theses practices as new normal.

It doesn't reduce the requirement for evidence, but it's a very realistic "why" when it comes to why they haven't produced any physical evidence.

Of course intelligence agencies would never do such a thing as have one of their officers pretend to be a whistleblower in order to trick the news media into publishing their propaganda. /s