The entire block of business will receive that Credited Rate. But there will be many different Rates of Return on CV for that block of business due to the various ages/health ratings/riders chosen/years policy has been in force. The IRR is just a result of the Credited Rate, which is why agents always speak in terms of credited rate since it is the most quantifiable part of the equation.
And for 5.5%-7.5% you can expect 3.5%-5.5% in the short term, 4.5%-6.5% long term (as in 25+ years). Assuming the policy is fully overfunded, decent health rating, and from a decent IUL carrier.
The entire block of business will receive the credited rate only if the insurance company uses a portfolio rate approach. Some do but many use a new money/old money approach like most, if not all FIA companies.
I recall that Aviva, back when they were called that, were promoting a 17% cap index for new business and illustrating very high rates. They were also renewing existing policies with a 10% cap.
I think that's unfair and maybe even deceptive.