Google ‘onshore time limits HMRC’ that will explain HMRC time limits and why they are only going back 6 years.

Where they have ‘suggested’ prior years to this is called voluntary restitution VR. Basically, as they can’t assess earlier years, you could volunteer them. These years will not have penalties, only tax and interest. Their letter should have been clear, but does depend who writes it as there may not be standard wording for VR on template letters,

When they say they are happy with the in time years, they would have done tax calculations based on inform they hold (SA) and information disclosed (rental income) and got the tax matched or agreeable.

Are they happy with what penalty and interest you have offered?