I'm seeing that SRC rebuilds engines for those companies. That's a lot different than building new engines. I do see where SRC says they do the GM Performance crate engines for GM. I'm not seeing anything about regular production engines that go into new vehicles.
Just 'cuz you ain't heard of something don't mean it ain't so. I've heard that Kameltoe can cure both cancer & world hunger; but I rather doubt it. You're so right: rebuilding an old tired motor versus creating a brand new one is like comparing apples and armadillos.
Fact: Several years back, Production of new L31 replacement engines AND (L31's cousin the very popular) 602 CT race crates was moved from Mexico to Springfield MO; thanks, in part, to president Trump ending NAFTA and creating USMCA in its place.
I'll grant ya this: it's true that GM & SRC did not campaign all over popular TV & print media describing those changes. These dynamics were not spoon-fed to the masses. Dig deeper sir. Until this week, perhaps you'd never heard of SRC ? Well, consider this. You're not SRC's customer nor partner; GM is.
Perhaps it's also likely you'd never heard of SRI either ? Originally HQd at St Louis, SRI no longer exists as SRI (merger) but did supply all US auto manufacturers as SRI (Siegal-Robert) for decades; with injection molded plastic parts. But, if you'll look closely at the back side of any your trucks' front grill bowtie or gmc plastic emblems --- look close & ya just might find tiny "SRI" callouts that came from the SRI molds (I was there, sir).
But back to SRC ---
SRC proved its mettle to GM
et al loong ago with its history of quality remanufacturing; SRC proved it had adequate capabilities to properly build new engines as well --- so GM tapped SRC. These matters were spoken of in several trade publications-periodicals (and I don't mean boy racer rags). Perhaps you can access older issues of SAE's
Automotive Engineering ? Also, probably referenced in Business, Economics & Political publications as well. I've been employed within this industry (Mobility aka automotive manufacturing) for quite some time; as both a Tier 1 and Tier 2 supplier to GM, and most others, and some of their suppliers. Same for decades-long membership in ASQ & SAE w/ certs; end TMI.
It's been a Global biz for a loong time. There are so many, many companies supplying auto parts or services to OEMs that neither you or I can know em all; nor keep up with who's in bed with who.