Simple, if I say my anger doesn’t last for more than a minute, then would it be true to say that my anger will last for an hour? No, because I said it wouldn’t last for even a minute. If God says his anger will not last for an age, then it logically cannot last for eternity either.
If this is not convincing for you then might I draw your attention to Micah 7:18
He does not retain his anger forever(ad),
because he delights in steadfast love.
Or as Rotherham puts it,
He has not held fast, perpetually, his anger,
for, one who delights in lovingkindness, is he!
You see ‘ad’ carries the sense of future continuing on and on, practically perpetual. The Concordant Literal Version translates it,
He does not hold fast His anger for the future.
For He delights in kindness.