Was it listed under this before? I have a feeling it did, but in no way should it be classified as a theme park!
When Merlin bought the Tussaud's Group back in 2007 (or may have been 2008), Warwick Castle was initially placed within the Midway category of attractions. I was working at the Castle at the time and remember sitting through a presentation explaining that the Castle was being placed in the same group as the Dungeon attractions and Madam Tussaud's. However, for various reasons it didn't stay in that group and was moved across to the Resort Theme Park division - whilst I don't know exactly when this change took place, I do know it has been grouped with the Resort Theme Parks since at least 2014, possibly 2013.
Having Warwick Castle within the Resort Theme Park division isn't something I agree with personally, however, from what I can tell, placing it in this category has allowed Merlin to allocate more funds to the conservation and development of the Castle than they otherwise would have done. Warwick Castle is also part of the sale and leaseback agreement Merlin made with Nick Leslau (a deal which also included Alton Towers and Thorpe Park).