It's been an interesting debate on here, and Big Jim Slade and SmokeyTA in particular, and several others too, have nailed the argument as to why Kevin Sinfield won the Golden Boot this year.
As for Fylde Warrior, I would love to know who this person is. It seems that he or she registered on RLFans specifically to post on this topic, and in doing so has made a number of posts that border on libel.
I would love to know why you do this and what your motivation is.
Why not come out into the open?

Did Sinfield play on his own for the Leeds Rhinos? No wonder they've won that many SL titles from 5th

His international credibility is a subjective opinion, yours is he lacks it. Most people would probably think that your opinion lacks credibility, especially the corner you have painted yourself into regarding England and Leeds Rhino’s especially has led to some fairly incredulous opinions that you have put forth. But both my opinion on your credibility and yours on Kevin Sinfields are irrelevant because it is impossible to say what would have happened because they didn’t play, it is unarguable fact, that Australia’s decision not to play, and the disappointingly potical proposal from NZ did deprive Kevin Sinfield of the opportunity for international success against the major southern hemisphere sides and as such it is clearly correct to say that it wasn’t his fault he didn’t have success against those nations this year.

Sinfield did experience international success this year by the way. The international game doesn’t start and stop with the Australians, however much they, and their fanboys believe it does.

Perhaps he was denied. He's won MOM the last two times England have played New Zealand over here. If we could've had a 3 game England vs NZ series their's a good chance he might have been Man of the Series.

Who awarded him Man of the Match in the game played in England?