How do they define collapse? The criterion in the article seems to be "building platforms occasionally" vs "not building platforms anymore"?
Building platforms is a strong argument there's some organized society (since any such action would require coordination), but the opposite does not have to apply (not having platforms does not mean there isn't a society).
Article's logic:
* Article assumes common knowledge of a collapse, and doesn't bother proving that.
* So when did the collapse happen?
* Well, the inhabitants were building statues during and right after contact.
* So probably afterwards.
That 'afterwards' could be right after inhabitants stopped building platforms, but it doesn't have to be for the article's logic to hold! If this is not the case (as you claim), than this just pushes the collapse date forwards from the point they stopped building (after contact). It can't push the date backwards (before contact), because they were building stuff previously (article's actual argument).
So if the assumption you mentioned doesn't hold, the article's basic argument - collapse after contact, not as a result of ecological catastrophe - becomes more likely.