I was misremembering from ~ a year ago from a skeptoid episodeThat's actually not true, fractional-wave antennae are absolutely a thing; example: Quarter-wave antenna - Wikipedia. Actually there are even fractal antennas which can radiate efficiently over a ton of RF spectrum.
Anyway, the need for an antenna isn't the limiter IMO. The real limiter here is the amount of RF power you need to send any signal at all from something so small that it fit inside a hypodermic needle. To get that maximum 5G coverage radius, you'll need to be able to transmit at least a few thousand feet. You're gonna need several hundred milliwatts to get there.
https://skeptoid.com/episodes/4735
the antenna still has to be physically large enough to capture the wavelength, and this is a limit set by physics, not by technology. On the order of about a centimeter is the lowest bound, which is far too large to be injectable. Pet chips range from about 1 to 4 centimeters in length and about 7mm wide, fat enough that the needle used to inject them is huge and terrifying, nothing like a medical hypo.