Hi magpie,
There are many versions of “down alternative” materials (see post #10 here) but they are all somewhat similar to the down alternative materials that are used in many mattress pads which are versions of polyester fiber (which would tend to be more dense than the same materials used in a comforter which aren’t designed to bear weight). They tend to be soft yes but how they feel in any specific mattress depends on the amount and specifics of the material and on how they interact with all the other layers and components in a mattress because every material and component in a mattress will have some effect on all the other layers and components in a mattress. For example the same type of mattress cover may feel very firm if there were firmer layers underneath it and may feel very soft with softer layers underneath it but it does provide a soft “hand feel” to the mattress.
Our mattress is certainly in the “soft” side of the range although softness and firmness is always subjective and relative to the person so some people may find it firmer than we do.
Phoenix