Everyone claims they received a decisive mandate, because a mandate is in the minds of people. If they think you have a mandate, then you do. So after every election, the winner claims to have a mandate, because they keep hoping that everyone else will act like they do, and therefore that they will.

Objectively, of course, you're right.

Some politicians actually talk about what they're going to do in some detail before election and then have some validity in claiming a mandate for those things after winning.

Others talk utter BS, make a million conflicting promises, loudly deny stuff that they fully intend to do etc. Those people do not have a valid claim at a mandate.