To be fair though both Hamilton and Alonso have concrete evidence of being able to challenge for or win races and world championships when not having the fastest car, whereas, due to Vettel having the fastest car for the last 2.5 seasons, he does not have this sort of evidence.
In 2008, Hamilton won the world championship in what was at very least a slightly slower car than the Ferrari or at least that is the general perception. Alonso also won a race in Japan, in his Renault, which was quite astonishing at the time. In 2009, as soon as Lewis had a car that was anywhere near capable of winning, i.e. Hungary, he won the race and by quite some margin.
I know a lot of people may not share this perception but I think in 2010 Hamilton's performance was quite remarkable. Alonso's 2nd half comeback was also remarkable but he did make quite a few uncharacteristic errors in the first half of the season (similar to that which Hamilton has in 2011). But in 2010 Hamilton would have somehow won the world championship in a car that was never really as fast as the Red Bull and that was without doubt slower than the Ferrari for the first and latter part of the season. This was obviously partly down to Vettel having such bad luck but also, Hamilton lost 18 points in Barcelona, which would have changed the outcome of the world championship in itself. He lost another 12-15 points in Hungary through a gearbox failure. He also was very unlucky in Singapore. I cannot remember any other major mistake than what happened at Monza, but suffice to say, he was my driver on the season personally.
Some of his performances, notably at China, Spain, Canada, Silverstone and Spa, even though he didn't win all of them were truly on another level and even those races he won, he didn't necessarily have the fastest car.
Vettel may well be able to put up the same sort of challange that Alonso and Hamilton can for the title in what is not the fastest car, but as of yet, there is no evidence and therefore Alonso is right in narrowing out himself and Hamilton.
I know everyone will say, look at Vettel's win at Monza 2008, but this was in the rain and the rain always brings unpredictable results - I am not taking anything away from that win but it does not give enough evidence.