There is no simple definition of immunity. It's not a binary condition. The human immune system is complex with multiple mechanisms at work. Long term immunity tends to depend more on memory cells than circulating antibodies.

Note that it didn’t say immunity. The quoted number is saying the rate of hospitalization and death among the vaccinated is 90% lower than it is among the unvaccinated. It’s a simple calculation. Group a had X hospitalizations, group b had Y. Adjusting for population sizes, X is 90% lower than Y.