Writing a general install engine like that is indeed hard. If that’s what people claimed they could do in a weekend more than an eye roll would be justified, such as hails of laughter. Such a reaction would be justified even if they said they could do it in a month or two.

However, if someone just claimed they could write a better installer for one specific program of theirs only that was better for that one specific program than an installer built with one of those general engines, that is not so unreasonable except for the in a weekend part.

I once worked for a DOS and Windows utility software company that wrote an installer for each product, and I was usually the guy that ended up doing the installer and it wasn’t difficult. If however they had decided to try to make an installer engine product for other companies to use I would have worked hard to not get assigned to that project.