I also think it's unfair that childcare/preschool is such a financial drain. For the wealthy/affluent it's not such a stretch but for those who can least afford it it makes things really tough. For those parents they don't really get that luxury of "choice".
So for those who keep their kids back a year are more likely to be wealthy and gaps emerge.
Preschool and schooling should be equal and universal, kids start with those who are the same age so a more "equal" footing, so no one is disadvantaged or falling behind.
Bit of a moot point anyway if the study is valid as there is apparently no disadvantage to starting "early" anyway