Here is the summary they included in their post:
> NFTs don’t prove ownership — ordinary legal contracts are just as good, and in fact better since they are heavily regulated and can be taken to courts for a central resolution when problems arise
> Blockchains (which NFTs rely on) are in the best case no more secure than ‘normal’ relational database storage systems, and in most cases are less secure
> Cryptocurrencies have no innate value and are only useful as a pyramid scheme, something big crypto influencers are knowingly using to profit from those who don’t realise it
> Both cryptocurrencies and NFTs cause high environmental damage, and there are no viable systems to deal with this problem due to security considerations
No one claims NFT proves ownership of entities outside of the used blockchain. But within the chosen blockchain, they do prove ownership of that particular NFT. That's a fact.
Saying that blockchains are less secure than a relational database storage points towards not understanding immutable ledgers with shared state at all. They both have different use cases, that much should be obvious if you understand the basics of both blockchain and relational databases.
Things that people value has value. That value is decided by what people think the value is. I don't understand art, especially not "high-art", but somehow people value it, that's fine by me, people like different things. But then I agree, cryptocurrency "influencers" are horrible. Follow the people developing the software instead, those are real and honest people (mostly).
And finally: no, cryptocurrencies and NFTs doesn't cause "high environmental damage" as those are concepts, not implementations. One implementation of consensus that is used in cryptocurrencies is Proof of Work, that is harmful to the environment, at least current implementations. There are other implementations of consensus, like Proof of Stake, or Pure Proof of Stake, who have their own drawbacks but have almost no environmental impact (compared to Proof of Work).