My guess for the reason in signing two centers ahead of Giles:
They really thought the knee injury was worse than anticipated. Knowing how this medical staff operates, or at least how they present on paper, they misjudged his injury. It seems like they're misjudging quite a few injuries, which is perhaps a topic for discussion at another point in time. Point is, they didn't think they'd get anything from Giles. Now, his knee could be a timebomb, and we won't know until we know, but for the time being, he seems to be a lot healthier than he was at the start of the season. And because he's able to get those consistent minutes, he's showing what he's capable of.