If you want to authenticate an individual email sender then that sender would sign their email. Just like with paper mail. Sure you can try to determine if the sender is legit by looking at the postmark on the outside of the envelope but that is not what it is for. The result will and can not be reliable.
SPF is Sender Policy Framework.
Something like "Originating Entity Policy Framework" might be more correct.
Posted article uses "Sender" for the user, not the entity. Authentication inside the entity is the entity's responsibility. SPF is only concerned with verifying that the mailhosts offering to deliver messages on behalf of entity are allowed to do so.
Email being a distributed system, the responsibility of identifying the sender is shared, and if implemented property works well.
The job of the client and recipient server is to verify the domain of the sender, according to rules defined by the sender.
The job of the sender server is to authentify the user account.
Implementing things properly is hard...