It would make more sense when water levels are already high. But then there are indeed people assigned to keep an eye on the crucial infrastructures .
It's not the flow itself that matters here, the rise is how they found out that there was a problem. The problem is that the sluice was set to manual operation instead of automatic, for an unknown reason, and stayed open when it should not have.
The guy would not have noticed a rise, what he would have noticed is the sluice staying wide open for a long time.
Also, the water rose 32cm in the canal, not in Amsterdam center. The locks were closed in time to prevent water from rising in Amsterdam, where 32cm could have been a problem. The water level is supposed to be mostly constant in the city center, it doesn't follow the tides.
The water does not necessarily "flow back" after a tide either, since most of Amsterdam is below the level of the sea at low tide. It must be pumped out.
edit: canals and baby rivers are lower indeed, they are pumped out and locked
https://www.tide-forecast.com/locations/IJmuiden-Netherlands...