How was he a sign to many?
Rashi: They saw his afflictions, and they worried lest similarly occur to them - "v'Hayah Yechezkel Lachem l'Mofes" (Yechezkel 24:24).
Radak: Many were chastised when they saw that I was punished for the sin. I am a sign that You punish for sin!
Radak citing Ibn Ezra: Because I relied on You, and You supported me, I was a sign to many who did not believe in You.
Malbim: Many considered me to be a wonder - my Gevurah and success is not natural, rather, a wonder.
What is "Machsi Oz"?
Radak: Even though I flee, due to the punishment for my sin, You are my strong refuge, that You will return me to my kingship.
Malbim: This is connected to the coming verse (refer to 71:8:1:2).
