Carbon can't be mechanically driven into iron - it must "disolve" in a high heat environment. The chemical combination of iron and carbon forms an alloy we call steel (carbon content over about 0.05%). The folding and heating process exposes more area of the iron to carbon and, thus, increases the rate of absorbation. Case hardening uses the same chemical process, but doesn't expose as much of the steel to the carbon rich casing material. The hazard of mechanical introduction of carbon is that any carbon not absorbed (or contaminate material) will be included in the final metal as a very weak spot. The larger the inclusion, the more detrimental to the strength of the base metal. Removing all extranious material and arriving at a known and controled carbon content, along with avoiding cooling shrinkage voids is the holy grail of steel making. Old time craftsmen did some very amazing work considering the tools, ores, and knowledge they had to work with.
Wootz has been replicated and documented. It produces a nice blade steel, but nothing special compared to modern alloys. Of course, it would have seemed magical to anyone used to using an iron/low carbon blade. Its claim to fame was being workable at low enough forging temperatures to not burn out the naturally occuring carbon and, so, leave you with a fairly high (for the time) carbon alloy. I believe the trick is a very narrow level of mangnese (I could be wrong on that alloying component). Anyone else remember the reports on this?
