That's some serious density and heat resistance!
While I'm still a big fan of the 'DIY washer method' (there's no metal seed phrase backup, remotely as cheap) - since a coinplate does not cost $300 USD, I may consider it in the future for a very 'heavy' (packed with good amount of coins) Bitcoin wallet.
Though if just your own house burns down (not a huge apocalyptic sized fire), you should have redundant seed phrase backups elsewhere.
Buried under the basement / in the cement sounds like a good idea.
Yeah, your house footings even if it was a fire or what not probably aren't going anywhere any time soon, so cementing in concrete is a decent idea. Plus, no one's going to be digging that up while you're there, unless someone pulls some Prison Break esque plan on you, but that's incredibly unlikely.
However, while it's nice that this product exists, and I know the costs have been figured out a little more since the OP, I can't help, but think I could just go to my local metal works shop, and get something of similar quality for much less, and wouldn't need to ship it. Obviously, you could argue that your local metal works could be using lesser metal, but you could potentially test it yourself to assure that. So, this product is decent for those that can't find any other means, but it's still rather expensive when you factor in almost everyone has access to this sort of thing locally at a reduced cost.