The point is that if it is able to be made small enough to fit through a syringe, the range will also be correspondingly smaller and the excitation field will need to be exponentially bigger. (think antenna sizes) Notwithstanding that the smallest commercial device (which implies reliable manufacturing systems) is still over 1mm square. That's bigger than can fit in a needle. The Hitachi vapourware from yesterday would be great but that was only 0.15mm square and there was no datasheet available for it on Hitachi's website either, therefore, there's no commercial development of that technology yet hence; vapourware!
It might be "not impossible at all" but, it appears to be completely impractical at the current state of bulk manufacture. Bulk because you will need to implant a host of the devices if you want any to survive the extremely hostile environment known as the human body. Also remember, the higher the frequency you go to, the better the body is at absorbing the applied RF and using the energy as heat which means that anything subcutaneous or deeper has an even less chance of receiving an excitation field and almost no chance of transmitting anything that would even be able to escape the body to be received by anything, even a skin contact antenna!
Not impossible; yes. Implausible at best and impractical for the most part.