Them winning isn't a shock; they seem to have popular support. Them being allowed to win is surprising though.

Not Dutch, but German and I can comment on our voting system: It's way harder to fuck with numbers.
We are in a single time zone. Voting starts in the morning of a Sunday (work free day for almost everyone, except essential workers like police/hospital and stuff like parks/restaurants). It ends at 6pm. In the last years (couldn't find 2021, since the article was written before that) the final result was announced at the following times: 2017 5.25am, 2013 3.15am, 2009 3.35am.
Basically less than 12hours from the polls closing every vote has been counted and no one can start counting before polls closing (same for mail-in ballots, only opened at 6pm).
While we do have mail in voting they have to be there before we get any numbers. You cannot just discover a random bag of uncounted votes.

And we don't use electronic voting. Piece of paper and then everything is counted by hand.

Of course we do have vote manipulation, but it's usually a single person doing shit.