As "His limbs and head were stuffed in bags in the living room" they do have some idea about the cause of death (e.g. that it's unlikely that he committed suici

As "His limbs and head were stuffed in bags in the living room" they do have some idea about the cause of death (e.g. that it's unlikely that he committed suicide or had a sudden heart attack). They don't know the exact mechanism of death yet, but even in the absence of anything else, cutting off the head is usually sufficient. For an investigation the video of the only person entering his apartment is perfectly good probable cause to say that they have a video of the suspected killer. Convicting the killer requires much more evidence, but that's going to be many months later, currently all the listed claims of the police seem perfectly reasonable for this stage of investigation.