AC Milan are 5 points clear at the top of Serie A. They're a decent side.
They are decent, although they have a bunch of players that aren't eligible for the CL this season that are starters in the league which make them a bit stronger, like that ever loved Van Bommel.
Even so, there's a general consensus that the Serie A is going worryingly downhill and is behind - in no particular order - the Prem, Budesliga and La Liga in terms of quality (and is definitely last in terms of the pace of the game). Inter won the league 5 years in a row till Jose left, never the sign of a strong league (see Lyon in years past). Haven't they lost one of their CL places to the Bundesliga?
All the Italian teams in the CL finished 2nd in their group, some barely; Roma got dumped out on Tuesday quite comfortably, Milan yesterday, and it would be no surprise to see an Inter team that've been a pale imitation of the ruthlessly efficient Special One side last year go out to Bayern next week, being a goal down and playing away.
This isn't to downplay Spurs achievement, but bar Inter last season (and only last season because they weren't that great in previous years), beating the big Italian sides hasn't been a daunting dask. Even when we beat Milan 2-0, with Maldini, Kaka and all those famous names playing, they weren't at the level they were in previous seasons.
Fulham (no offense, Fulham fans) utterly smashed Juventus last season.