You do still need that.
The broadcast from gpio thing is very low quality and produces square waves which interfere with everything
But yes, if you look at high-speed Pi GPIO with a Rigol it looks more like an EKG readout than the thing you might see on a logic analyser. Smoothing it enough to feed a line-amp is very lossy.
GPIO's usually looks quite squarey if you don't introduce parasitic reactances into the circuit with your test setup.
A spectrum analyser has probes!?!? This might be where I am going wrong... But the bench scope is largely in agreement about the distinct phases of a cycle at RF freq.