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.
For any product there is always a buyer. Accordingly, there is a corresponding demand for products, such as coinplate, even at declared prices. For people like you, who are able to make an coinplate's equivalent with the tools and materials at hand from the nearest shop, coinplate seems overpriced. But it should be borne in mind that not everyone has such skills and the desire to actually make seed phrase saver like coinplate by hands. It seems to me that the coinplate and analogues of other competitors are aimed at such buyers. I almost forgot to point out that they are also ready to expect delivery (buying locally is incomparably faster) and neglecting the safety of personal data, such as payment details (if you pay not in cryptocurrency) and delivery address. Buying locally leaves no "trace" that you bought the coinplate and respectively, you are the crypto owner.