Firstly, 'having a horizontal stabilizer is a source of inefficiency' is not an argument, it is a fact (one that might be a premise in an argument, but see below.) As you know this, how did you get from seeing that I said a certain argument is unsound to supposing that I am disputing this fact?
Maybe you think it is the only way that argument could be unsound, which brings us to the second point: the argument I am commenting on is not 'having a horizontal stabilizer is a source of inefficiency, therefore canards are more efficient', which would not even be valid. It is, instead, the argument that canards are more efficient because the conventional horizontal stabilizer usually produces a downwards force (incidentally, this is not always so [1].) While this may seem an obvious conclusion at first sight, it tacitly presumes a sharp separation of concerns which does not hold in practice.
The argument 'having a horizontal stabilizer is a source of inefficiency, therefore tailless designs have greater efficiency' (which was not made in the post I was replying to, but which is sometimes alluded to) does not hold up any better, on account of the compromises in making a stable and controllable tailless airplane (at least without active stability augmentation.)
[1] For some conventional airplanes, with the GofG near its aft limit, the horizontal stabilizer will produce an upwards force at low speeds (without being unstable as a consequence.) This happens to be the case for many gliders. A while back (and possibly now lost - at least, I have not been able to find it), there was an interesting article (by Wilhelm Dirks - co-founder of DG Aviation, I believe) explaining why, in practice, this cannot be exploited to get a little bit more performance out of a sailplane.
Maybe you think it is the only way that argument could be unsound, which brings us to the second point: the argument I am commenting on is not 'having a horizontal stabilizer is a source of inefficiency, therefore canards are more efficient', which would not even be valid. It is, instead, the argument that canards are more efficient because the conventional horizontal stabilizer usually produces a downwards force (incidentally, this is not always so [1].) While this may seem an obvious conclusion at first sight, it tacitly presumes a sharp separation of concerns which does not hold in practice.
The argument 'having a horizontal stabilizer is a source of inefficiency, therefore tailless designs have greater efficiency' (which was not made in the post I was replying to, but which is sometimes alluded to) does not hold up any better, on account of the compromises in making a stable and controllable tailless airplane (at least without active stability augmentation.)
[1] For some conventional airplanes, with the GofG near its aft limit, the horizontal stabilizer will produce an upwards force at low speeds (without being unstable as a consequence.) This happens to be the case for many gliders. A while back (and possibly now lost - at least, I have not been able to find it), there was an interesting article (by Wilhelm Dirks - co-founder of DG Aviation, I believe) explaining why, in practice, this cannot be exploited to get a little bit more performance out of a sailplane.