en.wikipedia.org/wiki/Imperial_Regalia_of_Japan

Reliable reference works describe the Imperial Regalia as three objects: the mirror Yata no Kagami, the sword Kusanagi no Tsurugi, and the jewel(s) Yasakani no Magatama. That means an “additional piece, an octagonal mirror” is wrong—the mirror is already one of the three sacred treasures, not a separate fourth item.

The sentence is also wrong about who first received them in myth. Britannica states that the Three Sacred Treasures were first given by Amaterasu to her grandson, while Jimmu is described as a later descendant who became the first emperor. So Jimmu is not the original recipient in the traditional account.

In short: the standard myth is Amaterasu → Ninigi → later descendants including Jimmu, and the regalia are three items total, with the mirror already included among them.

Multiple sources contradict the claim that only people “instated to Emperor status” can view the regalia.

Reporting on Naruhito’s accession, The Washington Post states that the treasures are seen by “the emperor and high priests.” A separate reference on the enthronement ceremony likewise says the regalia are generally seen only by “the emperor and a few Shinto priests.”

So the article's wording is inaccurate because it excludes priests, who are also described as permitted viewers in reliable accounts.