The dude is claiming to know 100% how the ship system works, yet we saw another streamer find a crashed ship on a planet and then proceed to fix the parts needed and took off in it. Now I am not sure that ship was better then his first ship, but I think the guy claiming to know how 100% of the ship upgrades work is malarkey.
I don't want to sound rude, but have you read what I quoted? He's saying that better ships become available only if you buy one better than the one you own, and it works for crashed ships too. I see it like some RPGs level scaling, like "you can buy lvl 6-9 items, if you buy a lvl 8 sword new lvl 8-12 become available", and the same goes for crashed ships: you can only ship better than yours, but only once you actually own them the level of the next available ship scales up.
Basically: if you don't buy new ships because you say "yeah, that one is pretty just like mine and the only difference is those two more empty slots", then you'll never get to see anything much better for sale.
Oh, by better I mean with more slots, considering how those are the only real difference and allow the player to forge their own unique ship.
Of course, this is what I get from his statements and my only intention is to share what he's saying. Honestly, I don't seen any reason to doubt him, he's still experimenting with some stuff, and if nothing else we now know that some mechanics need to be discovered and are not so clear. Nothing wrong about it.
A new piece of info about crashed ships
The crashed ships follow the same rules as purchases, yes. So the crashed ship will be the same +2 module improvement as a ship that an NPC would sell you would be.
So theoretically, if you didn't want to pay for new ships, you could just repair the crashed ships you find during exploring and go about it that way! It's much cheaper!