I'm not a rust or systems programmer but I think it meant that as an ABI or foreign function interface bitfields are not stable or not intuitive to use, as they can't be declared granularily enough.

C's bit-fields ABI isn't great either. In particular, the order of allocation of bit-fields within a unit and alignment of non-bit-field structure members are implementation defined (6.7.2.1). And bit-fields of types other than `_Bool`, `signed int` and `unsigned int` are extensions to the standard, so that somewhat limits what types can have bitfields.

Across binary libraries ABI, regardless of static or dynamically linked?

Dynamic linking is not something you do in general in Rust. It's possible, but the compiler currently does not guarantee a stable ABI so it's not something one generally does.