I don't see how... the PJ, based largely on flawed understanding of numerous aspects, had decided that the child was dead and the parents had covered it up (with the help of just about everyone under the sun).
They were simply not looking for a possibly living child. They weren't actually even looking for a potentially dead one, either. They were just banking on one or other of the parents cracking and confessing.
If they'd stayed, there was no guarantee that certain officers wouldn't find some new half-baked "evidence" and the whole thing would have gone from bad to worse. Amaral's next steps were to get "members" of the Smith family, in all likelihood just Martin Smith and wife, over to make a new statement - which would again be in Portuguese in reported speech. Oh, and to carry on looking for a fridge, possibly one with an undateable trace of unidentifiable blood in it. Job over.
I'm not surprised that some people thought that they were fleeing: the tabloids (and sometimes the broadcast media) had been leading the public to believe that they were guilty, after all the PJ "knew" so. At that time, no one knew what was fact and what was fiction.
Is that any different from Scotland Yards understanding seven years on? They aren't looking for a live child, that much is very obvious.
People forget that the McCanns were already contemplating fleeing from Portugal the moment they realised the PJ were going to make them official suspects. Didn't Kate admit in her book that Gerry wanted to load up the hire car and high tail it over the border to Spain?
The truth is they only delayed their exodus by a few days so as not to appear guilty. The PJ made a huge mistake in letting any of them go until every possible avenue had been examined.
The conduct of certain members of that tapas group leaves much to be desired imo.