> they simply don't count too premature (under 22 weeks or 500 grams, iirc) and therefore don't consider them in the metrics

Nope, that's not the reason.

> The U.S. infant mortality rate was still higher than for most Europeancountries when births at less than 22 weeks of gestation were excluded.

https://www.cdc.gov/nchs/data/databriefs/db23.pdf

Statistical hustling is common, but the most usual form is that the US excludes uninsured individuals to make itself look like it belongs in the developed country cluster.