David Frum, who wrote the article, is a unrepentant neo-con who's never seen a war America should avoid. For these reasons, he's been opposed to Trump since before he ran for president. The President has a right to defend himself against attacks by anyone including a whistleblower. The accused in America are allowed to face their accused. The whistleblower can't hide behind secrecy laws when their accusations can cause a president to be impeached or for anyone to have to face criminal charges. The purpose of the whistleblower law is to prevent the whistleblower's job from being terminated or facing discrimination by the government organization they work for. It was not set up to prevent accused people from defending themselves nor to block their testimony in a civil or criminal trial or impeachment.
Pure ignorance, Alan. Impeachment is not a criminal proceeding and the same rules do not apply. No right to face accusers, no ban on hearsay evidence (not that there's a complete ban in criminal proceedings anyway), no right to call one's own witnesses, no right to have a lawyer present. If you insist on drawing a parallel with criminal proceedings, impeachment is like a grand jury. It is NOT a trial. At a criminal grand jury, the purported defendant has NOTHING to do with it and the proceedings are secret. The house's impeachment process has been wide open (as it should be) with dozens of Republicans participating, questioning witnesses, and with (limited) subpoena powers. So the GOP and Trump have nothing to whine about.
And if Trump is so interested in have the truth come out, why has he forbidden 4 aides from testifying? You and I both know the answer.