By "crypto functionality" the author means talking to a blockchain, not doing cryptographic operations. Metamask, the example given, is a browser extension tha

By "crypto functionality" the author means talking to a blockchain, not doing cryptographic operations. Metamask, the example given, is a browser extension that provides access to the Ethereum blockchain by injecting a JS object into the web page. It provides a wallet and transaction signing and broadcasting. Web crypto API's are quite different.