The Guardian's title is a bit misleading I would say.
"[A UN spojesperson] can confirm that he is retiring today. He informed the UN in March 2023 of his upcoming retirement, which takes effect tomorrow. The views in his letter made public today are his personal views.”
So he steps down because he is retiring, not because of the genocide.
You are correct. He has served the UN since 1992 and he did not step down for Yemen or any genocide. It is just his retirement letter.
Also this is mentioned in the article:
He was criticised for posting support of the boycott, divest, sanctions (BDS) movement and accusing Israel of apartheid – an accusation which he repeated in his retirement letter.
Mokhiber said he had not been made aware of the review.
He said: “Israel lobby groups regularly harass and complain about UN officials who speak out on Israeli violations, but the UN is used to this tactic, so I would be surprised if any such ‘complaint’ went anywhere.
[...]
After the death of the Palestinian-American journalist Shireen Abu Akleh, who was almost certainly fatally shot by an Israeli sniper, Mokhiber posted on X, formerly known as Twitter, that Israel was “whitewashing the cold blooded murder of its own citizen”.
The post continued: “No accountability. Just an official cover-up. A pattern of supporting #impunity that goes back 75 years and includes covering up war crimes, crimes against humanity, ethnic cleansing, apartheid & state terrorism.”
We'll come up with a new word for bombing thousands of children! Not "genocide". Too icky. We'll call it something friendly. Like Public Cleansing. Or Forced Relocation.