To me, the most relevant parts of your article are below and clearly explains why they aren’t appearing.
Here is another relevant article: https://www.lawfareblog.com/executives-privilege-rethinking-presidents-power-withhold-information
“The Office of Legal Counsel deemed the subpoena “invalid,” and Butler said Ellis had “been instructed not to appear.”
“Michael is respectful of the legislative branch and will cooperate with a valid subpoena,” Butler said.
Last month, head White House counsel Pat Cipollone said that White House officials would not cooperate with the inquiry, arguing that it “violates fundamental fairness and constitutionally mandated due process” and privilege and immunity for the executive branch of government.”