I think that's a fair assessment on their part however it's worth noting that your phone number does not serve as your account ID. It can be used to look up an account but there are caveats to that.
The lookups go through a secure enclave, the system is architected to limit the number of lookups that can be done, and the system has some fairly extensive anti-exfiltration cryptographic fuckery running inside the secure enclave to further limit the extent to which accounts can be efficiently looked up.
And of course you can also remove your phone number from contact discovery (but not from the acct entirely) but I'm not sure how that interacts with lookup for subpoenas. If they use the same system that contact discovery uses, it may be an undocumented way to exclude your account from subpoena responses.
The rest of what they say however is pretty spot on. The priority for signal is privacy, not anonymity. They try to optimise anonymity when they can but they do give up a little anonymity in exchange for anti-spam and user-friendliness.
So of course the ending notes of "use a VPN, configure the settings to maximise anonymity, and maybe even get a secondary phone number to use with it" are all perfectly reasonable suggestions.