Well I also put a separation between something being transhumanIST and something being A transhuman. So I even agree that any augmentation beyond normal human capacities is transhumanIST. I just don't think it makes one an outright transhuman. A bit similar to how I consider many things to be humanist, but one can be a human without being of or in any connection to humanism.
Gene therapy by itself isn't transhuman. Playing with genetic alteration beyond 'defined' human capabilities is. For example, one could genetically alter things to the point of growing two tails, and it'd be hard to not call it transhuman by that point, even if those tails barely change your capability otherwise.
But yeah it can get complicated. Personally, I think that if a human is genetically altered to both accept and infuse itself with tech in a seamless fashion, then it is already POSThuman. That's the general thing we see with the Collectors, some hints of the start of it with Peace Quarians, and Synthesis. Transhuman indicates a transitory state where there is more difficulty and elimination of the (previous) self (so Husks, or at least the partial husks, fit this a little better than Collectors), whereas posthuman is the eventual new self that no longer needs to give up anything. For the record, Collectors are a crappy form of postorganics. Harbinger sucks.
There's many things to compare to this. Humanism can be the Thesis, Transhuman can be the Antithesis, and well, Synthesis is the Synthesis. One clings to the familiar Thesis and struggles toward/is dragged towards the Antithesis, all for the eventual compromise, balance, and goal of the Synthesis. And that's just one way to put it.
This doesn't mean that the Synthesis is the outright ideal *state*. Just that it is the ideal *solution* for the *most people*. Such it is more IRL with posthumanism. If we all have tech as ingrained in us and our genetics, does that mean that the human race is technically extinct? At least with transhumanism, we see a (even if decreasing) attachment to the human, but we have no idea what would happen once the posthuman state occurs. Transcendence was a bad film, but it showed one of the more typical views of what could happen when a transhuman mind gets enough power to initiate posthumanity.