The obvious follow-up question is, given their limited resources, why were they spending some of it on closed-source code based on other people's commercial code?

What're you talking about, there?

The answer quoted above said they needed to spend time to "untangle it of some commercial code that was used at some point." That just strikes me as a bizarre problem for Canonical to have even had.

Only if your focus is open source. UbuntuOne was conceived of as a service, so to make the project go faster and solve problems commercial products were used. As you'd expect you then have to disentangle that code to release you code as open source - happens all the time in big proprietary code bases that are being open sourced.