The president was actually counted as number one before so that Anon must have read outdated information.
In the event of a President’s death, next in line is Vice President; after that, the Speaker of the House moves to the White House.
But that has not always been the order of succession. In the late 19th century, the protocol specified that after the Vice President, the Senate’s President pro tem would be next in line. That caused a problem in 1881 when President James Garfield was assassinated. His Vice President Chester Allen Arthur was himself seriously ill, but Congress was not in session, so there was no elected officer to be second in line.(Or third, if you count the sitting president as number one.)“As a result of this and other crises,” TIME later reported, “Congress in 1886 passed a new Presidential Succession Act, making the Secretary of State next in line after the Vice President.”
Then later the president change the line off succession again…