And you're basing this theory on what?
Tin is not a normal alloying agent in steel.
https://www.thebalance.com/common-steel-alloying-agents-properties-and-effects-2340004
From the Kreg website:
Steel Kreg Screws are case hardened — a process forms a tough outer shell (or case) to make the screw tough, but keeps the core softer to prevent the screw from becoming brittle. That means you get the best of both worlds: a strong screw that won’t snap.
Torsion strength is needed to drive the screw. Shear strength is what keeps it from failing under an applied load.
Here are the independent lab test results showing the strength of Kreg Screws. Granted, these values weren't measured hanging cabinets, but the values tell me the screws are more than strong enough. In all tests, the wood failed before the screws did
https://www.kregtool.com/webres/Files/KregJigHDForceTestSheet.pdf
Full disclosure; I don't work for Kreg, have no financial interest in Kreg, and only use their screws for rough cabinetry. I am impressed with how strong they are, though. Never had one break on installation, and have never had the screw fail in a joint.
I wouldn't hesitate to hang cabinets with any #14 screw of the appropriate length, (as long as it didn't come in an assortment from Harbor Freight...)
Of course, all of this discussion is around whether the OP can save $5 on a box of screws...