If you can put a satellite into orbit that's definitely NOT "too small to be tracked"
However, as stated in TFA, the satellites turned out to be quite trackable.
Live tracking: https://platform.leolabs.space/catalog/L19943
> the SpaceBees have shown themselves to be easily trackable by the Space Surveillance Network, as well as by LeoLabs, a California-based company that provides orbital data to commercial-satellite operators and others in an effort to prevent collisions.
I'm not sure if "Space Surveillance Network" is the same thing as "Space Situational Awareness", but it does sound like the US Government is capable of tracking these things.
Radar also suffers from atmospheric attenuation, so there's a tradeoff between frequency, power, wavelength, etc.
I don't know what systems they're using to track LEO objects, but it's certainly within the realm of reasonable.
A decent primer: http://faculty.nps.edu/jenn/Seminars/RadarFundamentals.pdf
I'm going to guess here, but you might be thinking that orbits are permanent and space is empty, so once you observe the point of insertion, you can calculate object's position anywhere in the far future. This is very wrong. Orbits are unstable and decay with time, due to residual atmospheric drag, solar radiation, interactions with Earth's magnetic field, the irregularity of the Earth's shape itself, and other factors. All of these combine to the need for continuous active monitoring of space around the Earth, and recalculation of trajectories of existing satellites. A satellite needs to be trackable in order to be able to update its trajectory, as it changes naturally.