Is swearing by Allah the Almighty that my opponent has no right whatsoever to mention in the site, and that I built upon my property to absolve myself, a matter of doubt or suspicion in religion?

As long as you built on land that is not yours, you have lied in your oath and committed a 'Ghamus' oath (a deeply sinful, false oath). Interpretation will not benefit you, for an oath is based on the intention of the one administering it (the judge), by scholarly consensus. What is obligatory is to repent to Allah. The 'Ghamus' oath does not require expiation according to the majority of scholars, but expiating for it is more cautious and safer.