That matches my understanding.
AIUI …
Every year until death they will assess IRMAA based on your tax return from 2 years ago unless you file form SSA-44 to explain why a different number is applicable to your case.
Bottom line being:
While your year to year income is pretty stable, the SSA’s default behavior is close enough. Whenever you have a significant disruption in income or change in family situation you can file form SSA-44 to make the change in IRMAA assessment take place a year sooner than the default delayed 2-year look-back provisions would have.