The steel Ford uses is hardly "high strength", this is really just marketing BS as their own documentation indicates that it has the yield strength of normal low-carbon mild steel. They used to use 36,000psi steel for their frames, A36 steel or similar; but they started using 49,300psi steel, 1010-1030 or similar and called it high strength by comparison. Both of these mild steels can be easily welded with standard ER70S6 MIG wire which has an as welded yield strength of 65,500psi. No after weld heat treating is required, but care should be taken to reinforce the repaired area as there was obviously a defect in that area that failed.

The "high strength" steel being used on the frames has nominal thickness of 0.087" for LD frames, 0.100" for HD frames, and 0.110" for HD/HPP frames. Back when Ford used 36,000psi steel for some of the 2009-2014 frames, the nominal thickness was 0.150", and the same year 49,300psi frames were 0.100" and 0.110" nominal thickness by comparison.

I do agree that no Ford dealer will do this, but they aren't welding shops and they don't all have big collision shops where frame repair is a common practice. A Ford dealer will only replace the frame or frame section because that is the only type of repair that is officially approved by Ford. Other types of repairs really need to be looked at on a case by case basis so there's no good way to offer official guidance outside of saying "repairs are not recommended, just replace the whole frame or frame section".