I agree when it comes to the matter of an entirely fresh engine, but I think it's also possible that many aspects may just be those from the prior iteration of the engine, maybe dialed up, whilst the most impressive/complete aspects of an engine upgrade are the ones which are actually showcased. At least at this early stage.
For example, the shader quality, skin physics, and animation are astounding in the trailer. It would make sense that those are the aspects of the Hellblade 1 engine which have been steadily improved upon since the last patch was released for the initial game, and that those could continue without any kind of significant paradigm shift. Lighting, on the other hand, is presently in a state of flux, and I feel quite safe in assuming that it's the aspect of their engine which is most perplexing to them right now (i.e. best practices haven't really been demonstrated industry-wide at present, so there must be a lot of toing and froing over the right balance of techniques in a hybrid lighting engine. Assuming that's what Hellblade 2 is, of course.) So, just dialing up the fidelity of the lighting techniques from Hellblade 1 might make the most sense at this early stage, hence a lack of apparent RTRT.