There is a lot of bad law on this site.
For one thing, he says that the 20th amendment providing for precertification of the Vice President and President and what happens as a result of the fact that "the President elect shall have failed to qualify" only applies until the President is sworn in.
Lots of people argue something like that however there is no authority for the proposition and in my opinion, it is not correct--under the Constitution, the next thing that happens when the President is found to "have failed to qualify" is that "the Vice President elect shall act as President until a President shall have qualified".
Where we are today, Biden should be acting as President until Congress decides what happens next.
His description of operation of the 25th Amendment is just obviously wrong. When a President is removed from office for any reason, the Vice President serves. Reason that doesn't apply here is that Obama isn't President because he was never "elegible" to hold the office. Biden (arguable) doesn't become President until a real President is designated and then dies or is removed because there isn't anywhere the Constitution says that a Vice President who is acting President under the 20th Amendment automatically becomes President--his position is waiting until Congress either designates some person elected President or takes some other action appropriate under the 20th amendment to determine a President.
Congress cannot impeach him because the power of impeachment extends only to President's and since he is not eligible to hold the office, Obama isn't President (and therefore cannot be impeached).
Some of his acts are probably inoperative because he isn't President; some acts are operative because his action as President isn't required.
That wasn't the point of the link.