Dictionary.com defines as to begives a few pronoun used relatively in like manner of relative pronouns —senses for that, who, whichas etc. Under item no (2) it is said to mean "a fact that" and thus exemplified., including this one:
- She did her job well, as can be proved from the records.
- a fact that:
She did her job well, as can be proved from the records.
Our instant example usesAs functions analogously to relative pronouns like aswhich as a pronoun. We may make use of 'anaphoric it' for emphasis but that changes the orientation altogether., Asthat without, and itwho in the first place; it serves the dual function of pronoun and 'relative subordination'subordinator.
Inserting it would completely change the meaning.