I think one way to interpret it would be to divide the total amount of income tax and national insurance payments used to fund all public services/government by the amount of people living in the UK. That will come to amount 'X'.

The phrase 'you don't become a net contributor to the state until you're a higher rate tax payer' could then be interepreted as only those whose total amount of national insurance and personal tax payments is more than amount 'X' are actually paying disproportionately more to support lower income tax payers.

As an example (not accurate figures)

• The cost of providing public services/government/state facilities is say £1000 per person (total cost of providing those services divided by the population).

• The average person pays a total of £500 in national insurance/tax, therefore doesn't pay enough overall to pay for their 'share' of state services.

• Higher rate tax payers on average pay a total of £1100 in national insurance/tax therefore cover their 'share', plus have a net (left over) amount of £100.