That part is true. It is usually extremely difficult to prove. It's why the case on whether it's genocide can take years. And why people saying the ICC hasn't said its 100% genocide yet aren't arguing in good faith. They said it's plausible, which is already huge. Anything more wasn't realistic, as it will take them most likely literal years to finish the case out, but we can call it as we see it before then.

In this case, it's still okay for everyone else to say it because we don't have to prove it legally, and it's pretty obvious to the eye and ear with the mountain of evidence given by South Africa. Luckily, Israeli government officials and soldiers have said openly many statements basically proving that they want to do a genocide. They've called Palestinians animals, compared Gaza to Amalek, said they need to erase the Gaza strip from the earth, said there are no involved civilians, been encouraging another ethnic cleansing through emigration as well ("If there are 100,000 or 200,000 Arabs in Gaza and not 2 million Arabs, the entire discussion on the day after will be totally different."), etc. Those quotes and statements have always helped me feel more comfortable calling it what it is.