Well when they could get the XF working with both IQ3 and 4 including power share, aperture control and shutter control it would certainly also be possible to make the IQ3 at least control the aperture and shutter of the XT including power. They chose not to do so! Don't make it more complicated than that.
Pardon, but it is more complicated than that.
The IQ3 (an extension of the IQ1/2 platform) had the appropriate electronics and controllers to interact with the XF. In that relationship the XF is doing all the heavy lifting when it came to power and lens control. Regarding power: the XF is doing the power management itself, with the IQ3 simply sharing its battery to the XF as the XF requests. Regarding aperture/shutter control: the XF itself does all the management and can accept requests from the IQ3 to make a change to those settings. The XF is a co-equal brain working together with the IQ3.
The X-Shutter is NOT the same thing as an XF from that point of view. It is significantly smaller and lighter and can't do much of anything in and of itself. The IQ4 was built specifically to have the required hardware and firmware to act as the power supply and brains to an X-Shutter. It's a good first approximation to say accurate to say the X-Shutter is an "accessory" to the IQ4 platform; the brains are all in the IQ4.
Now look, Phase One has brilliant engineers, and I'm quite sure they could have worked around all this if it was deemed mission critical. Maybe each X-Shutter could have had more components added to it to be more autonomous. Maybe they could have offered a service to take IQ3 backs and add the required hardware to allow it to be the brains of an X-Shutter. Or as I suggested earlier they could provided an alternative path such as an external controller with separate battery. But all of those options would have carried significant cost (depending on the option, costs such as development complexity, time, weight, size, component costs). My point is only that it is "more complicated than that". There wasn't a switch sitting there ready to be flipped that would make an IQ3 control an X-Shutter.
I 100% understand the annoyance/frustration or even anger that an IQ3 cannot control an X-Shutter. You bought a system not that long ago and now there is a new thing that it's not compatible with. I even share that frustration, as we have a ton of IQ3 clients that we'd love to sell an XT with X-Shutter lenses to.
The IQ3 was the last in a generation of technology (the IQ1/2/3 backs all being mostly the same platform with each new IQ offering incremental improvements and sometimes a new sensor), and that means it becomes out of date sooner than previous backs in that series. I remember very similar (and justified) frustration from owners of the P65+ (the P+ backs all being mostly the same platform with each new P+ offering incremental improvements and a new sensor). If you bought a P30+ it was many years before something was made that didn't work with your back, but if you bought a P65+ it was only a couple years. If you bought an IQ180 it was many years before something was made that didn't work with your back, but if you bought an IQ3 100mp it was only a couple. I would expect that those buying (or upgrading to) an IQ4 will find that it will be many years before anything is made that is not compatible with their system. But (and feel free to link to this in several years) when Phase One is on its 3rd or 4th incremental iteration of the technology in the IQ4, that will likely not be the case.
Generational leaps leave people behind, and frustrate customers. They are also the only way to stay relevant.
I get your frustration. I get their decision.
It's only partial consolation of course, but the IQ3 can be traded-in toward an IQ4, which, in addition to being ready for an X-Shutter offers quite a few features/advantages including Automated Frame Averaging (another feature only possible because P1 totally redid the architecture and made a generational leap).