Pretty sure kernel doesn't have to fsync on close. In fact, you don't want it to, otherwise you're limiting the performance of your page cache. So fsync on install for dpkg makes perfect sense.
I didn't say it synced. The file is simply "written" and available at that point.
It makes no sense to trust that fsync() does what it promises but not that close() does what it promises. close() promises that when close() returns, the data is stored and some other process may open() and find all of it verbatim. And that's all you care about or have any business caring about unless you are the kernel yourself.
