Never is a long time, but it's also a theroetical.

The sun will eventually engulf the Earth rendering this argument irreelvant.

It works both ways, though. I was responding to a claim with an implicit "never" possible. Like you say, such a claim is theoretical, and I wanted to ask how that person knew it would never be possible.

Smell, taste and poison are all matters of physical molecules and how these interact with human physiology. We already understand these to some extent. We can already build devices that can detect some of these things to some extent. We have no grounds to say it's impossible for a human-level ability of it to be engineered. (Note that lack of positive evidence against X is not the same as positive evidence for X).