Keller swam for the United States at three Olympics before he joined the march against its latest election result.
Tokyo is battling a surge of COVID-19 cases that prompted the national government last week to call a state of emergency.
A senior member of the International Olympic Committee has said he ``can't be certain'' the postponed Tokyo Olympics will open in just over six months.
The man reportedly leaped over a 10-foot fence along the heavily-defended Korean DMZ, dodging guards and barbed wire to escape Kim Jong Un's regime.
Athletes arriving in Tokyo for next year's Olympic Games will be exempt from the 14-day isolation period.