OK, that should suffice. A Google search for it wasn't finding much in the way of results and the Greater Anglia site doesn't acknowledge Brentwood!https://www.nationalrail.co.uk/stations/brentwood/ says it’s in London Fare Zone 9
Zones 7-9 are not true Zones in the sense that Zones 1-6 are. They should be, but it was all done as a bodge. In other words, not all zones are treated equal.but also what reason is there to believe that TfL and National Rail would disagree as to the zone of a station?
If the origin is outside Zones 1-6, the Travelcard is deemed to be "out boundary". The bodge to include Zones 7-9 (which initially only applied just beyond North West London) is to amend the route field, rather than the destination.
In other words, a Travelcard from Brentwood is an outboundary Travelcard; furthermore it is 'also available at' Zones 7-9.
Some sources state that the validity of Brentwood being in Zone 9 is 'unadvertised', though now that it's included in the NRCoT, that's clearly out of date.
While I don't believe it is intended that a non-stop split is possible at Brentwood, pragmatically it absolutely should be accepted, but notwithstanding whether it is or isn't intended to be valid and/or whether it should or shouldn't be accepted, if anyone was faced with being charged an excess fare, new fare, penalty fare or a prosecution, there should be sufficient evidence to say that the (sloppily worded) rules of the NRCoT are met; if nothing else on the basis that where there is any ambiguity over a contracutal term, it should be ruled in favour of the customer.
However a Wickford to Brentwood plus Brentwood to Zones 1-6 combination might not guarantee a stress-free situation, in the (admittedly unlikely) event of a ticket inspection.