It was poorly worded, but they are actually right. What they meant is, "30% of intermittent decentralized energies within a net designed for centralized power production, without any grid investment". In this case, the claim is right, and you could push 40% at most, with caveats.

Which is, in fact, used as a point in favor of wind : it is often argued by those opposing it that you can't produce much wind-base energy without storage ; but empirically, wind alone can be 30% of your power without storage solutions, or 40% with only minor grid investment and some (or quite some in the worst case) curtailment.

Obviously as you said, there is plenty of ways to reach higher. And the word "renewable" was poorly chosen, since eg. hydro or geothermal are renewable but fit in a "traditional" grid with no problem. Obviously they meant intermittent ; the figure holds for wind. Solar is significantly lower because of its daily cycle, you do need storage.

30% is a commonly quoted share of intermittent generation that can be part of a stable, healthy grid without storage, at todays technology. I’ve read it quite a few times from articles linked here, including presented as “studies”.

However l’m not going to dig for references because:

• highly depends on conditions

• rapidly changing with technology

• I don’t entirely believe it

It is a useful number, though, to give some approximate scale. When all the silver-bullet people say “but muh storage, and the wind stops blowing”, the answer is that we’re working on it, plus the grid ought to be able to absorb around 30% even without storage