What would they do that? The fact that Xmarks is closing doesn't mean they would have sold it for a few bucks. Not to mention that Pinboard is run just by two guys, as far as I know without any external investment.
I admit that I did not investigate it thoroughly enough. As you concluded, they may not have the money or the man power to acquire Xmarks. Ignoring these vital issues, it appears that they have the necessary passion and expertise in bookmark management.