The ECDC guidance in that link is against second boosters which would be the 4th shot in the series.
The CDC's guidance is that everyone has a single shot of the "updated" vaccine which has higher efficacy against omicron/later strains -- no matter how many vaccines they've previously had.
Since Europe generally had higher vaccine uptake, their level of protection is already at a higher level than the US - so the differing recommendations seem to suit the respective populations pretty well.