Yes, that is absolutely correct - these cards are not part of anyone's collection, they are only accessible in the preconstructed deck event, and they will not be accessible to anyone after that event ends.
However, as @Aelien pointed out, these cards are not only functional (granted, none of them are super complex) but several of them are also animated, including at least Mox Pearl, Mox Sapphire, Sol Ring, and Black Lotus (not sure about Ancestral Recall). Given that the preconstructed deck event is a short-term promotional event, it seems odd that they would devote animation efforts to these cards unless they have at least an inkling that they may bring them back for a more sustained outing at some point.
In case everyone missed it, there are some real vintage cards - Black Lotus, Moxen, Ancestral Recall, Sol Ring - in the current free Arena event (an "Ashiok versus Elspeth" pre-constructed deck event to promote Theros: Beyond Death). It was quite startling to open a starting hand with Ancestral, Black Lotus, and Underground Sea in the Arena client.
I would not be surprised if they bring a Vintage Cube to Arena at some point in the next year or so, though obviously the reliance on bots would be a weakness.
@brass-man I know there is the "Stitcher" App - it's referenced at the end of some of the podcasts I listen to (e.g., "Find this podcast on iTunes, Stitcher, and wherever else you find your podcasts"), but I'm not sure if there's a separate submission/aggregation process for that app.