It is dishonest if they represent their intentions as this and no further when they aren't.
But I don't think that is what people are doing - they are indicating whether or not they support the current proposals. That doesn't seem dishonest even if, in an ideal world, they might prefer something different. If that something different is sufficiently close to what is proposed to be a likely amendment, then fair enough to suggest that change. But if what someone would ideally want is something pretty radically different I don't see it as dishonest to indicate what their views are on the current proposals, but park their ideal scenario until or unless this becomes the topic of some future debate about a change to the law.
Was it dishonest for a gay person in 1967 to indicate that they supported a change in the law to make homosexual acts lawful, even if in their ideal world they would wish to marry someone of their own sex. I don't think there is any dishonesty at all.
The time to debate some hypothetical future development of assisted dying is for just that - the future. Currently we are focussed on what is currently proposed.
