In any case he would already have the type rating on his license before commencing line training. What he appears to have been missing was a final line check. It says somewhere that the FO joined in 2017 so even with Covid was quite probably an experienced Airbus pllot converting to the A350.
It is a serious administrative error like flying with an out of date medical and will rightly attract some attention from the authorities.
The amount of initial operating experience required also varies widely between companies and of course countries. The USA I have heard tell makes IOE pretty quick. Some companies I know of in Europe can take many months to get people through their line training. Of course not everyone passes or achieves the required proficiency in the standard time.