What is load on an antenna? Is it safe to short the antenna port?
The section you quoted appears to be referring to the antenna connection of a receiver.  Yes, you should be able to either short the antenna input or connect 50 &Omega; there.  The point seems to be that you don't want ambient RF affecting the noise source used for cryptography, since that give someone else the ability to affect it.

50 &Omega; is presumable the impedance of the antenna input, so that would match it.  It practice it matters little whether the antenna input is shorted or terminated at its impedance.