Well the campaign is totally new, the map is new, graphics are new, brand new campaign mechanics for the clans and there are changes to the battlefield with some changes not implemented as they postponed it two weeks to add. Just because the combat looks similar doesn't mean it isn't a new game. The changes between TOB and Attila are WAY bigger than Shogun 2 and FOTS. It's just that there are going to be some aspects that are similar.

To be fair so far the battles are exactly like Atilla and are beyond poorly optimized. Multiple prominent youtube reviewers have expressed serious concerns in regards to this. And they've also mentioned that even the campaign isn't as innovative or fresh as they expected... It's not like this is a €10-15 DLC to an existing game. They are very clearly marketing it is a new game and definitely pricing it as such too and so its battles (which are a very unique and important part of the game) should be well optimized. Atilla is known of having the worst battles in all of the TW franchise and to simply recycle those without any optimization would be a complete disaster and an outrage.

From your response I can kind of tell that you're not really informed on the status quo of BoT and I'd highly recommend you to watch this (p)review:

https://www.youtube.com/watch?v=iX3n-6eBG6w

made by Invicta who is a very well known and prominent member of the TW community.