No, that means either they, the terrorist, were already in the country or came in recently.
Would you rather stop them from getting in or let them in and hope you can stop them?
In other words would you rather ingest a poison and hope the doctors can counter it or have someone stop you from ingesting the poison?
From what I understand, you were speaking of hypothetical thwartings that you believe have happened, i.e. We can't claim that no terrorist attacks were attempted because we don't hear about those that were thwarted. But since we don't know the details of these possible thwartings that occurred, how do you know the origins of these possible attackers? How do you know that the ban would have stopped them? How do you know they weren't American citizens, or from Saudi Arabia, or that they weren't turned away by our current vetting process?
We do know that there have been 0 deaths from refugee terrorist attacks in our country since 1980. We know that of the 3.25 refugees welcomed into our country, only 20 have been convicted of committing or attempting to commit terrorism. That's a pretty darn good track record.
We also know that of the terrorist attacks that have occurred, the vast majority of the perpetrators are American citizens or from countries not included in the ban (Saudi Arabia, Egypt, and Pakistan). So, the ban wouldn't have stopped them.
There is simply no evidence that we are "ingesting poison", as your analogy argues. There is even less evidence that the ban would prevent the ingestion of poison.
The ban is a draconian solution to a problem that barely exists, and worse, it doesn't particularly solve the part of the problem that does exist.