There is no way to figure out whether standard suites are available from the website, therefore the FD manager was probably right, calling him a liar is completely uncalled for.
Hotels as well as airlines overbook their capacity. If hotels are happy giving you a senior suite while listing junior suites as available on their website, we are only talking about one scenario.
Additionally, well-organized hotels block suites for Platinum members in advance, so if the suite you are talking about was blocked for another Platinum member, it is certainly still available for booking, however not available for you at check-in, if the other Platinum member did not check in yet.