I thought that the minimum value cwnd could assume is IW, but looking at RFC 2581 that isn't true: "Upon a timeout cwnd MUST be set to no more than the loss window, LW, which equals 1 full-sized segment (regardless of the value of IW)." They even explicitly call out what I erroneously believed, so you are right -- I apologize!

If you really have a tin cans and string physical layer, Google's larger IW could be more disruptive to other connections on the link: A vanilla TCP connection would ramp up cwnd from 1 segment until congestion is observed (in the slow-start phase) and then grow cwnd conservatively (after ssthresh is first set). If congestion would be observed at a cwnd value less than 10 segments, then starting with IW at 10 segments could be very disruptive to others sharing the connection.

Mind you, this argument feels very... academic. As you pointed out, Google's connection would converge toward fairness anyway. (Unless so little data is actually transmitted, then the connection probably isn't open long enough for that to happen.) And most shared links don't saturate at 12 segments. I'd guess that a high-capacity link would only be at risk if it has a lot of connections (so that every connection has a cwnd not much greater than 10) and there are always many (albeit, short-lived) connections to Google always being created (which could appear as fewer long-lived connections with a constant cwnd of 10, more than would be fair).