Booked and just completed a stay under that 250EUR credit promo, but AmEx THC (so I would imagine equivalent in their revenue mgmt system).
Note that this is a halfway between THC and FHR in my mind because they offered complimentary breakfast as an extra amenity (only CC Gold right now).

Chose to have one breakfast in Porter (it was OK, ordered some avocado toast with eggs and they gave me a plate with a scoop of avocado and a side of eggs and told me to get the toast myself from the buffet), and one in-room dining.
Skipped the Duchess breakfast due to time on checkout.

I inquired about the credit before I arrived with the W Ambassadors or whatever they market it as, and they confirmed the same as your DP, that it could not be used at Porter nor Duchess.

However, on check-in, and confirmed in writing via chat during the stay, they said that the credit could now be used for both restaurants, and it was successfully written off at checkout. So, that may tilt the needle in your favor if you're on the fence, FWIW.

I liked the vibe of the Duchess significantly more - the Porter is almost like a cross between a nightclub/lively bar and a steakhouse, which I'm not sure how I feel about. Very lively though, and excellent drinks (taste and presentation)

Crossing between the two buildings was less annoying than I thought. Wanted a swim and had to book a reservation through the spa (says it costs money but you won't be charged).
Note that the black on black theme in the pool, which is in the bank vault, is a bit terrifying to do any actual swimming in - hard/impossible to see the wall.