What's the benefit of balanced tonearm cables?

Balanced line and single-ended (RCA connections) are inherently incompatible. If you have one then its not the other. There is no such thing as pseudo balanced- that would simply be single-ended.

He may have meant a balanced line with RCA termination. That is how I run my set-up. This presupposes that the RCA input of your phono pre is balanced. In my case, with a transformer input, the XLR and RCA inputs are connected in parallel with a ground lift switch. Lifting the ground converts the RCA input to balanced. One could argue that the asymmetry of the RCA connection would introduce some differential noise. I would argue that the effect would be small at best.

My reason for this arrangement is convenience. I have a Rega style tonearm where the cables exit the mounting post and have to fit through the hole in the arm pod on my Nott. TT. Not keen to unsolder and solder XLRs every time I have to pull the tonearm.

If an RCA is used as a balanced connection, which is dicey due to the grounding scheme, the result is that it will be prone to noise pickup because of the imbalanced introduced by the connector itself.

IME using a RCA plug to terminate a balanced phono line (twisted pair with separately grounded shield) is not as problematic as you suggest. In an ideal world one would want to use XLRs. Where that isn't practical using a RCA termination won't be the end of the world. I am assuming the RCA jack connects to a "floating" transformer winding or a true diff. input.

If an RCA is used as a balanced connection, which is dicey due to the grounding scheme, the result is that it will be prone to noise pickup because of the imbalanced introduced by the connector itself.

IME using a RCA plug to terminate a balanced phono line (twisted pair with separately grounded shield) is not as problematic as you suggest. In an ideal world one would want to use XLRs. Where that isn't practical using a RCA termination won't be the end of the world. I am assuming the RCA jack connects to a "floating" transformer winding or a true diff. input.

Balanced signals have greatly reduced common mode noise,

Just to be precise, balanced signals can have common mode noise, maybe even a lot. It's that balanced circuits reject the common mode noise and only amplify the differential signal.