I think: "How confusing: 'SNDR' is not sender; why not
'ORIGIN' or 'AUTHOR'"
Sorry, I am week in exact naming.
And also: "How is this going to protect us?"
No, it is not to protect us. It is to protect message path
related envelope addresses against hiding or erasing. It can
replace SRS after some time but otherwise it is off-topic.
In order to verify the authenticity of the VIA and ERRORS
TO claims,
a cookie or something similar should be present.
If You can use cookies You dont need SPF at all. So it is
off-topic.
z2