Isoionic is what you get in pure water, it’s for most cases a hypothetical state. Isoelectric is closer to reality due to presence of counterions, other electrolytes, and pH active substances.
The trick is to create enough binding opportunities, either by increasing the amount of negative charges on hair so that positive charged conditioning agents can electrostatically bind (like magnets of opposite poles attracting each other) or by increaseing surface. Someone here on board posted a link to a paper explaining how this actually works a few months ago. The problem with hair is that there are always positive and negative charges and these attract or reject each other. This results in higher or lower hydrophilicity/lipophilicity and accessibility/’swellability’ of the ‘layers’ (constituted mostly of proteins and fatty acids).
The different effects are working against each other and hence, there is no easy answer/solution. What seems to me as most favourable and which also reflects the prefered use of acidic conditioning is the denser structure of hair with ‘layed down’ keratin scales at low pH.