Yes, I have heard and read about the Modal argument. It has at least two issues:
1) It assumes a timeless perspective. The statement that "Gold was first found in California in 1990" is possible, but only from a timeless perspective. Once gold was found in 1848 (or whatever year it was), that statement became "not possible", and the statement that "Gold was first found in California in 1848" became necessary, because it is now (at the present moment) no longer possible that "Gold was first found in California in 1990." This principle is called "The necessity of the past."
Further, to say (on this day) that "it is possible that gold was first found in California in 1990" falls into the same "irrelevant" category as most tautologies. It is certainly not a basis for any rational argument.
The author's response to "changing the past" is also invalid. He immediately says that, since God exhaustively knows the future, God knew about the prayer request and acted beforehand to fulfill the request, such that the past isn't really changed, even though it may have appeared to.
Thus, this doesn't address God changing the past. The past is clearly necessary, and God is unable to change the past. That statement did not change.
And, it assumed that God has exhaustive and definite foreknowledge, something that has not been demonstrated.
Finally, the article rejects (very early on) what is clearly true (from a secular perspective): Propositions about the future are either a) neither true nor false, or b) their truth or falsity is logically unknowable. Yet, the only significant argument against considering this is "bad consequences" (we'd have to rethink logic.) So, the author tacitly assumes determinism from the beginning, even as he tries to prove determinism.
And, that doesn't address humans. This is somewhat related to #1, in that we are temporal beings. God may have been able to act in advance to change what seems to be the past, but really isn't, but we are not. To us, past decisions are not contingent.
2) This argument weakens free will considerably. This, again, is due to timelessness. Ultimately, the author will have to make the argument that God's knowledge of a free will decision is logically after its execution, even though it is temporally before. However, because of the "timeless" view of modal logic, this is "acceptable", even though the contradiction is evident (and has been demonstrated at least since Aristotle.)
And, ultimately, the argument fails because the author as assumed an atemporal perspective to evaluate things that are necessarily temporal. If you can insert the perspective of time into this argument, I'd love to hear it, but this old argument fails the smell test up front, and ultimately fails in the area of logical consistency inside a temporal world.
Muz