Your analysis is correct, in that “verified” deterministic signatures obviate the need for a protocol like anti-exfil. However, the trade off that anti-exfil makes is that it does not require signing with multiple devices before knowing that the signature does not leak data.
Consider that without anti-exfil, you must sign and check every input with multiple devices before exposing the tx to the network. It is not enough to perform this validation after the fact; by the time you determine that different signatures have been produced, enough bits of your private key may have been leaked to allow theft either directly or by grinding the remaining bits.
Not using anti-exfil means that to achieve the same level of leakage assurance, you must sign every tx with multiple devices and verify the signatures before sending. That’s probably fine for an offline vault or cold storage, but it is neither practical nor supported by warm/hot wallets for typical send flows.
Anti-exfil exists to provide assurance for the common case of a single signing device. If you are prepared to sign and compare with multiple devices then you likely don’t need to use it. Like everything in cryptography there is a trade off between convenience and security; it is up to the individual to determine where on that spectrum they feel comfortable.
