Yes it is. Your last sentence relies on a misunderstanding on how air security is conceived. Screening is important but it is by no means everything, and every service in charge of security is aware of that. Without going into details, there are, unfortunately, plenty of things that ill intended people could do even after they have been security screened once, twice, or ten times.

Much of the security process relies on intelligence, profiling, and the attempt to identify any unusual or incoherent pattern. Trying to take a flight which is not one you are booked on would score maximum points on that front and would be considered in and by itself a major reason to worry about (and assess) the possibility of a safety issue.

Virtually all the people who have sadly managed to threaten the lives of air passengers since 9/11 were security screened, and unfortunately, it did not prevent them from sometimes managing to pursue their sinister goals. This is the reason why the focus is increasingly on intelligence and analysis (generalisation of API and PNR information sharing, suggestions to rely more on interviews/interaction as already done in some places like Israel, etc).

So while everyone would always hope that a passenger managing to board the wrong plane would be a mere mistake, anyone in charge of security will treat it as a major safety issue and also investigate whether this was indeed in error (as is clearly the case here) or intentional, and if intentional, what was the said intention.