Declaring an interest: we're Build4less, a building materials supplier.

EPS below ground isn't wrong in itself. Insulated raft foundations and ICF basements use it all the time. The difference is the grade: those jobs use denser EPS, usually 100 to 200, and most render system makers specify XPS or a dedicated perimeter board for the plinth because it takes up less water and copes better with knocks and backfill pressure. EPS 70 is at the soft end, so that's probably what's bothering the BCO.

I'd try to turn it into a risk discussion rather than a rip out. A few things that tend to help:

Ask the BCO exactly what the concern is (water uptake, compressive strength, or simply not matching the system spec), because the answer is different for each.

Go back to Baumit for a project specific letter rather than a general email, confirming their plinth system over EPS 70 graphite at your depth is acceptable, ideally referencing their system approval.

Put a dimpled membrane over the rendered face below ground before the gravel goes back, so the board is protected and water runs straight down to the french drain.

Send a section drawing showing the depth. If it's only a couple of hundred mm below finished ground in free draining gravel and never sits in water, that's a very different case from wet clay backfill.

You could also run the U value with a worsened lambda to show it still performs even if the board picks up some moisture.

I have 400 mm of EPS 100 under our slab, which was specified by our structural engineer. Who specified yours?

And yes, EPS is better underground than XPS. That's why it's used under railways and major road embankments.