HK–227: The SMG of choice for many corporate and
military security forces, the HK–227 boasts a retractable
stock, smartgun system, and integral sound suppressor.
,emphasis mine>
This isn't the only weapon with multiple accessories but specifically spells out one as integral.
If you read the mount definition in Core as meaning any accessory which comes with the weapon then why use the word 'integral' with the above sound suppressor.
My contention is that the mount definition was just poorly worded.
An accessory is only 'integral' if it says so.
Otherwise it takes up a slot.
To look a it otherwise is to allow illogical combinations.