You really have no idea what you're talking about, do you?
Compare Blackwater Park to Dream Theater or Symphony X and you will realise it can't be progressive metal either.
What the hell are you even trying to accomplish? Trying to label things to death (lol) and forcing everything into certain groups or boxes is, aside from unbelievably stupid, also extremely narrow-minded. The music of Opeth on Blackwater Park contains death metal elements. Your statement that it's 'musically not death metal' is provably untrue and 'very aggressive rhythm of death metal' is not an objective characteristic of a musical genre. Again, you have no idea what you're talking about.
If you're so inclined to be 'right', tell me what you think is 'death metal' then. For me, Opeth's BWP checks a lot of the boxes; Minor key tonalities? Yes. Growling? Yes. Distorted and down-tuned guitars? yes. Double bass in the drums? Yes. Chromaticism in riffs and chord progressions? Yes. Of course it sounds different than 'Altars of Madness' by Morbid Angel. Bands and musical genres are - lucky for us music fans - no boundaries that can't be crossed. Be glad we have stuff that transcends your narrow views of what a genre 'should' be like. Blackwater Park is a progressive death metal record and it can be that while being neither a true 'progressive metal' album, nor a 'death metal' album. It's my favourite 'death metal' album. That's it.