All seed phrase-related operations are now happen on the client’s side
How can this claims be verified, and is there a way to generate mnemonic phrase offline that can later be used with Blender Wallet?
Unless it's open source, it's not easy to verify it unless you pay experienced auditor (with JavaScript/cryptography expertise). Additionally, this wallet isn't compatible with BIP 39 (i tried it with result from
https://iancoleman.io/bip39). So there's concern you can't restore access to your coin if the service is down/dead.