There is no credible person or agency that says use low octane fuel for hybrid vehicles as a generic statement. It depends on the individual design.

So lets talk of individual vehicle manufacturer recommendations.

Toyota recommend use of regular 87 octane fuel in the Prius. The reason is it is a low compression engine. It doesn't need high octane fuel. So a litre of low octane fuel will be pretty much full of power delivering stuff you can burn, with little stuff that was added to boost the octane rating that may not burn.

Ditto for the Nissan Altima Hybrid, Ford Escape Hybrid, and Toyota Camry Hybrid.

I suspect it also applies to the Honda Civic Hybrid, but I don't know for sure.

Hybrid vehicles using the Toyota system have an engine tuned to the Atkinson cycle. Here the object is to have a long power stroke so you can extract as much energy from the burning fuel as possible. Unfortunately, if you have a long power stroke you also have a long compression stroke, and so would have to use high octane fuel and take other measures to avoid knock. In the Atkinson cycle, they hold the intake valves open for part of the compression stroke, to avoid high compression. The final result is a low compression long power stroke engine that only needs low octane fuel.

But as I said in the beginning, it is design specific. It could be different for a different design.

Pearl is a
2007 Driftwood Pearl Prius
Package "B" (everything but leather, nav, and rear camera)