This is correct. They share name and some furniture interchangeability but that's it.
There are some slight similarities but they are completely different actions and parts are not interchangeable.
Think of the Swiss Sig 55X guns as Modern Western Swiss AK. Its basically the finest built AK on the planet.
The SIGUSA 55x guns are a bastardized harbor freight version where everything is cheapened out. These sold for around $1200 with healthy profit. Its not a cheap gun to make right, hence why very few people bought the US versions, they are junk.
You can still buy real Swiss 55X guns new from JDI. They are generally $4K plus for rifles and 3.5k+ for Pistols (all are imported as pistols due to features and the import ban). here is a great example:
The one sort of exception is the the US Made Sig 551A1. Still a bastardized cheap version, but they at least attempted to make them more Swiss correct. Sadly they are now highly collectable and are going for north of $2k. They were blown out by CDNN when SIG liquidated them for peanuts.
If you have the scratch, Swiss guns are one of the best investments in the gun world. Amazing firearms and they will only go up in value over time.