Maybe this will help explain things
[image|2471149]
As we can see the periwinkle colored path is PCIe/NVMe which is what your drive is as it is reporting the higher level NVMe.
As far as your SSD’s size, lets take a deeper look as EveryMac listing [https://everymac.com/systems/apple/macbook-air/specs/macbook-air-core-i5-1.6-13-retina-display-true-tone-2019-specs.html|13” MacBook Air, True Tone (2019)]
[quote]This MacBook Air was available by default with either 128 GB or 256 GB of SSD storage but it also could be configured with a 256 GB, ***512 GB***, or 1 TB SSD at additional cost.[/quote]
So you have a real Apple system!