Nope, not correct.
They also need to guru 100% of the radicals.
That’s usually the case by the time one gets to 90% guru’d kanji, but in some cases (e.g. fast level) one has to pay extra attention to it.
You only need to Guru radicals to the extent that they are needed to unlock kanji. Strictly speaking, they are not required and you can level up without Guruing all of them.
Well, that’s what the rules and the dashboard say.
Where?
I’m pretty sure kanji are the only thing that matters since that’s all that’s mentioned in the guide and the only metric I see on the dashboard. However, I don’t remember any situation where I leveled up without getting 100% of the radicals as well so I’m not entirely certain.
But notice that there is no “Goal: 90%” in the radicals progress bar. It’s not required.
For example, level 51 has 1 radical and 35 kanji. The one radical (if guru’ed) will unlock one kanji. So if you fail to guru that radical, you only get 34 kanji. Guru’ing 32 out of those 35 kanji will get you to level 52.