The flight, itself, isn’t that long. Most normal men would be likely to “hold it” or go before it got off the ground. From that perspective, I get the speculation.
But, certainly, there are other possible ways to get the captain to leave his post. I don’t think spiking the drink would have been necessary. Most diuretics do not work instantly.
There was multiple legs without long breaks. Iirc, the co-pilot suggested he go since he was holding it since the last stopover.