Well now, it's not compulsory that former presidents attend inauguations.
The article cites recent exceptions.
And I don't recall that Richard Nixon attended Jimmy Carter's inauguration in 1977 (Gerald Ford, outgoing president, was there as a matter of course), despite his being the then-only-living former U.S. president.
I don't recall that Harry Truman and Lyndon Johnson attended Richard Nixon's second inauguration in 1973, even though both of them were still alive.
I don't recall that Harry Truman and Dwight Eisenhower attended Richard Nixon's first inauguration in 1969, even though both of them were still alive (Lyndon Johnson was there as a matter of course).
The history books don't say anything about Harry Truman attending Lyndon Johnson's (1965), John Kennedy's (1961), and Dwight Eisenhower's (1957) inaugurations, or about Herbert Hoover, who was still very much alive at the time, attending Kennedy's either.
Whoop-dee-doo.