So now that we have context, lets evaluate the current video.  I havent watched it since the thread came out, and wont again, so bare with me if I get details wrong.  First of all I would like to agree that MGS4 will not come to the Xbox 360.  But the reasons are these:

1.  Sony have signed (what was no doubt very expensive) exclusivity contracts with Konami.

2.  This far down the line, the game wouldn't be bought in sufficient numbers on a new console anyway.

Now here is what's wrong with his reasoning:

1.  There is no evidence that MGS4 is 50GB, simply that it is more than 25GB.  Kojima confirmed a dual-layered disc, not a full dual-layered disc.

2.  It is common knowledge amongst those who are versed in these topics that most high-graphics PS3 games have data re-used on Blu-Ray discs to compensate for lower maximum read times.  This is something the Xbox 360 doesn't need to do and on a large game like MGS4 could account for many, many GB's of data. 

3.  MGS4 is NOT compressed as is.  In fact it is much touted for having uncompressed 7.1 audio (something only a tiny proportion of the population can use).  On Heavenly Sword for example, audio alone took up 10GB according too the developers, and MGS4 is about 5-9 times the length of that game.  The audio of MGS4 compressed could literally save (and here I am speculating) over a dozen GB's of space.

So suddenly we are looking at a game that (while no doubt requiring more than one disc) may only be around 10-20GB on the Xbox 360, as opposed to up to 50GB (but probably much less) on the PS3. 

But there is a very simple way to prove this guys is talking absolute crap, and he provides it himself.  FFXIII.  By all accounts, this game was going to be HUGE (before going multiplatform this was always touted by PS3 fans as a reason it could NOT come to the Xbox 360).  For at least two (but probably more) years it was developed with ONLY the PS3 in mind.  FFXIII as a basic file will probably be much larger than MGS4 was.  And yet it will ship on no more than four DVD9's.  This will immediately and comprehensively debunk what this idiot is saying.

The first two reasons you have asserting exclusivity, is one that is constantly battled by people. There is no answer to either of them. We don't know if there is an exclusivity contract, and nobody knows how much it would sell. I agree that it would be a small number, but some seem to think it would sell at least 2M+. All in all these points are debateable. The other ones, (when he thought of it originally) were supposed to be undeniable/undebatable reasons. So i understand why he is talking about it.

1. By virtue of your own #3, the game is a full 50GB. However, it is uncompressed audio, and we also don't know the exact proportion of data it takes up. Was Heavenly Sword compressed? If so, then we have at least a  rough estimate of how much space it would take up. I also can't argue about space for sound because I don't know how much compression works. Does it cut the size in halves, fifths, tenths?

2. He did make a point about redundant assets, which would need to take up space on each 360 disc. He also said that MS reserves space on each disc for security reasons. I dunno, how much either of those points are true but they make sense to me.

I agree though, it could either be 2 discs to 8 discs, surely 17 is out of the question.