I'm not familiar with the specifics of these 600 or the accuracy of the claim, but the early Ottoman forces were a bit of a hodgepodge set of troops. Far from being the "exclusive" entity that tribalism is often understood as, the Ottoman "tribe" could and did incorporate all sorts of people who were not blood relations. This included a large number of Christians, non-Turkic Muslims and others, even if Turkic Muslims were the core of their force.

While I'm unfamiliar with the specifics of this claim it would not surprise me at all, given the constant shifting of allegiances in the Ottoman border territories, that some of the defenders during the siege had fought for the other side. This was a common feature of the Ottoman way of war during their initial conquests and given the extraordinary cultural and religious intermixing in Anatolia at the time.

Edit: /u/hollowegian gives a much better and more specific answer than mine.

The Ottoman defenders that fought alongside the Byzantines and Italians at Constantinople were led by a pretender to the Ottoman throne, Orhan.

Although it is unknown what relation he was exactly to Sultan Mehmed II, he was possibly the brother of the previous sultan, Murad II. To some Ottomans he had a better claim to the throne and for that reason he was kept by the Byzantines as a 'bargaining chip' to release when they felt it would benefit them the most. The chronicler Kritovoulos says:

Orhan, the uncle of the Sultan, of the Ottoman family, happened to be present there at the time [of the siege] and fighting on the wall with them, for the Emperor Constantine had him in the City and was treating him with much respect and honour because of his hopes. Orhan had been a fugitive for a long time through fear of his brother who had tried to kill him.

The Ottomans paid for the Byzantines to keep Orhan in Constantinople and when Mehmed II ascended to the throne in 1451, he agreed to an increase in the annual sum paid. When the Byzantines tried to put the price up yet again in 1453, however, Mehmed refused and used the insult to justify his stranglehold on Constantinople.

Nicolo Barbaro, a Venetian defender who was less attuned to Byzantine political manoeuvres, had this to say in his diary:

[Orhan, who] was in Constantinople in the pay of the Emperor, was guarding one of the quarters of the city on the seaward side with Turks in his pay, who had previously rebelled against their master.

It is known that Orhan died during the siege, but how is less clear, and the fate of his Turk supporters is unknown. Kritovoulos says that Orhan threw himself off the city walls as the Mehmed's troops were breaking into the city and that his head was cut off and brought to the sultan. Another contemporary chronicler, Doukas, on the other hand, says that Orhan donned a monk's robes and 'lowered himself through one of the archer's embrasures' to escape the city. Orhan was subsequently captured and beheaded. I'm afraid I don't know where the number of 600 comes from, but it is fanciful as the emperor's advisor, George Sphrantzes, says that there were 4,773 Greeks and only 200 foreigners defending Constantinople.