If you are talking about the Surface RT, that work was built on 32-bit ARMv7. Keep in mind Apple was the first to deliver an ARM64 chip to market (in 2013, while Surface RT shipped a year prior). Apple moved quickly enough on ARMv8 that other OEMs were caught off guard.
That said, Win 10-based WoA supports both ARMv7 and v8, but I’m not sure what state the WoA port at the time, and if it had been fully ported to ARMv8. Anything not ported to ARMv8 wouldn’t benefit the AS port.
As Erik Schwiebert mentioned in an interview, parts of Office are dependent on the .NET runtime, which also needed to be ported (and is one of those complex dependencies you mention since it includes a JIT engine). And don’t forget that VBA is another JIT engine. And as iPad version doesn’t support VBA, you couldn’t depend on the iPad version to catch all the stuff Mac/Windows did. Erik also mentioned in the interview that they were brought into the Apple labs back in March under NDA, so months prior to WWDC.
That said, all the previous work certainly cut down the cost of the work for sure. But Office in particular is a rather old beast with lots of nooks and crannies for work to hide. The thing still supports COM and OLE on Apple Silicon.
