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There are basically two viable options:

Renewables plus short-term storage (for ~6 hours, which is enough to shift production peak to demand peak) plus long term storage would be one.

The other is renewables plus nuclear and long term storage. Here you can get by -compared to a fully renewable model- with less renewables (although that part is financially speaking neraly irrelevant) and slightly less long-term storage. Nuclear base-load mostly eliminates the need for short-term storage and makes power-to-gas as long-term storage more efficient (electrolysers work much better economically when you can have them run most of the time, instead of needing a lot to use up peak overproduction while not running the rest of the day).

If you want to look at numbers (and different models) there's a big study of France' grid provider (from end of 2021 I think) about nuclear power models by 2050. And they assume roughly (they modelled more or less nuclear) 35% nuclear / 65% renewables.

Also we can assume a demand increase for electricity by a factor of ~2,5 when industry, heating (where it didn't already happen) and transport is electrified to get CO₂-neutral. So if you are going the nuclear route you would need (on top of a lot of renewables) nuclear capacities of more than 80% of today's demand (80% / 2,5 = 32%...).

Also if you don't already have high nuclear capacities available already you need to start building en masse preferably yesterday. Because to meet already agreed upon climate goals starting slowly now and burning a lot of fossil fuels for another 20 years until newly build reactors are ready will not work out.

And now compare this with reality: Basically every country talking about nuclear plans is still in some early planning phase, none of them are planning sufficient capacities and a lot of them are also still stuck in some imaginary and nonsensical nuclear vs. renewable discussion.

So to answer your question... He ist right because you either plan a sufficient amount of reactors right now or you don't plan any at all. And if you can't realistically pay the upfronted cost of a massive nuclear build-up right now, then that's a reason not to do it. But building just a reactor (or 3) to pretend that it will reduce CO₂ quickly until you have a better solution or can afford to build more reactors is just stupid bullshit. That's what a renewable upbuild is for, that actually helps reduce emissions within years, not in a decade or more when a nuclear power plant is build.