The Rendez-Vous album was pretty much made using the Elka Synthex, but clearly there are other synths there. So, to achieve the robot vocal effect without a Talking Pedal, there are only really two ways to achieve it:
A synth with dual filtering to allow for formant, vocal style effects
The Talking Pedal has been pictured in several studio shots and JMJ has talked about using it in interviews. The talking pedal is essentially dual filters that provide the formant vocal effects - and based on the time period, the original talking pedal was available.
So if you doubt that it’s not talking pedal and instead another synth, then you will need to find a synth of that era with dual filters that allowed for formant shaping.
A third way to achieve the effect is by advanced EQ-ing, which is essentially what a vocoder is doing in some respects creating bandpass/comb filters.
But as a Talking Pedal owner and having read a JMJ interview where the talking pedal was mentioned, I’m pretty certain it’s the EHX Talking Pedal. But do remember, the Talking Pedal needs a sound to process, the pedal will just change the formants of the sounds to give it the vocal effect. The sound going in will also have an effect on the final vocal sound.
For other demos by other people, as above, the effect is easy to achieve on many modern hardware and software synths and effects plug-ins with bandpass/comb filtering/EQing.