RF Circuit (what does capacitor do?)
The whole point of the DC bias network (TL3 and C1) is to present a very high impedance at the junction of the three TLs, in order to perturb the RF signal flowing through TL1 and TL4 as little as possible.

At these frequencies, the parasitic effects of every component must be considered. For example, even a tiny SMT capacitor has a certain amount of parasitic inductance in series with it that creates a self-resonant frequency at which its impedance is lowest. Above this frequency, the inductive reactance dominates and the impedance starts rising again.

My guess would be that the capacitor chosen has a self-resonant frequency at or above 12 GHz, presenting the minimum possible impedance at the node between C1 and TL3. This gets transformed by TL3 into the highest possible impedance at the junction of the three TLs.