I'm no expert in searching Rebrickable, but turning up the minimum block count to over 3300 seriously cuts down on the number of matches. I've tried variations on the names and creative spelling, but I don't think I necessarily turned up all Technic cars with enough pieces. How might I better check that?
First of all Rebrickable isn't the one and only place to share your MOCs.
Next, there are several requirements for adding new MOCs: it has to have an inventory, free/paid instructions and design should be owned by you (or by someone from collaborative project). So you cannot simply add a model, created by someone else, just to keep its inventory in your collection.
Model designers own their designs, so they may add and remove/hide their creation from Rebrickable. And this have happened before. It doesn't mean some model wasn't ripped off if the model doesn't exist on Rebrickable. And here is why:
First picture in your question is a model of "Aston Martin Vantage GT 3" by DugaldIC. It doesn't exist on Rebrickable. But it is the work of a person that isn't mentioned.
Next model is Gumpert Apollo IE. I couldn't quickly identify author of this creation. But it has been produced by Lepin which isn't reputable brick model manufacturer. Use of "Nifeliz" brand may be used to appeal to be more reputable, but I would bet both brands being manufactured under same roof.
Again, both models are pretty sophisticated and aren't something your average Joe could built. It takes time and experience to create something like that. Offering a number of BIG and complex Technic models suggest me these weren't designed specifically for this brand but were sourced quickly via other means. There is a chance that model author actually agreed to sell his design. I don't remember if it was CADA who offered this? But the fact was that manufacturer was creating models no matter if MOC author agreed to be paid some small fee or not.
Other red flags include the the lack of Aston Martin and Gumpert licensing on a box as well as model naming, like "Apello IE".