Yeah I thought that any Utd fans that were bitter (especially those still angry about it years later) about what the Sunderland fans did that day were just being over-sensitive, just like the Liverpool fans complaining about the Wolves fans on Sunday are being. Ironically Sunderland took more points off City that season than anyone else, and were routinely beating them 1-0 at home (4 seasons in a row) at the time.
On that same day at the end of 2011/2012, Bolton were relegated after drawing at Stoke, and the Stoke fans were mocking Bolton's plight as you'd expect. On the final day in 2008/2009, the Villa fans were pretty mercilessly mocking Newcastle who were relegated (to be fair most football fans in the country probably found that hilarious). It's all just football banter, dishing it out and taking it. When we were relegated in 2011/2012, I remember Wigan fans (yes they do have a few here and there !) mocking us quite brutally as it was mathematically confirmed, and I wasn't angry about it at all.