[qpw]

Although it's a little hard to grasp at first, the cards that have been dealt (provided they remain unknown) are still, for the purposes of our calculation part of the deck!

Consider it this way:

Suppose you take a complete pack of cards, cut it in two and select a card from one half.

Has the probability of it being, say, a club, changed because you spilt the deck in two before selection?

Obviously not.

So if you consider the part from which you didn't make your selection to be those cards that were dealt to other players, it is clear that the number of players dealt in does not affect your chances of making a flush (or, indeed, getting any other card).