If he did, then the consequence above stated seems to follow.

But I do not at all care to press this inference; it is no more secure than the premiss upon which it is founded.

Only it seems to me that the choice lies between two alternatives and no more; either Justin used our Gospels, or else he used a document later than our Gospels and presupposing them.

The reader may take which side of the alternative he pleases.
The question is, which hypothesis best covers and explains the facts.

It is not impossible that Justin may have had a special Gospel such as has just been described.