Your lawyer is the one to tell you. If you don't understand, ask him again. Also try and read the Notarial Act (if it is one) or legalisation for yourself.
But a "hold on the property" doesn't sound like ownership to me. The reference to 5 years might be to the period of time after which an innocent possessor can apply for a title based on prescription (Art 79 Ownership Act). But whether you are in possession is debatable. Act 16 might be a prerequisite for that. Alternatively it could be some limited form of title because if the building is not completed within 5 years it becomes illegal and liable to be demolished, so they say. I've no idea what the reference to two years might indicate.
It is possible to apply unilaterally for the contract to be performed and you to be granted ownership of the flat under Art 19 of Obligations and Contracts Act but the price must have been paid in full. Anyway that needs a court application not just a trip to the notary. My guess is that all that has happened is that your preliminary agreement has been legalised.
If the owner actually turned up at the notary and signed the Act your lawyer should have made sure you have priority over any mortgagee and that you had an indefinite title.
If you find out what happened it would be nice to let us know