に他（ほか）ならない is a grammar pattern used with verbs to indicate that something is ‘definitely (A)’, or ‘nothing but (A)’. Other common translations are ‘none other than (A)’, or ‘simply (A)’.
This expression is often used to emphasize statements, reasons, or causes.
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かすみさんがみんなに好（す）かれるのはお人（ひと）よしだからに他（ほか）ならない。
The reason why Kasumi-san is liked by everyone is simply because she is a good person.
こんな夜（よる）遅（おそ）くに訪問（ほうもん）してくるなんて迷惑（めいわく）に他（ほか）ならない。
Visiting me during such a late time is nothing but a nuisance.
Being a combination of the 名詞（めいし） (noun) 他（ほか） ‘other’ and the negated form of the う-Verb なる ‘to become’. This phrase literally just states that (A) will not become anything other than what it is, as the given definition is already the most accurate.
Caution - In formal situations, particularly formal writing,
に他（ほか）ならぬ may also sometimes be seen.
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Aチームがこの大会（たいかい）で優勝（ゆうしょう）できたのは岡村（おかむら）コーチのおかげにほかならぬ。
The reason why A team was able to win this tournament is simply because of coach Okamura.
我（わ）が社（しゃ）の商品（しょうひん）が人気（にんき）なのは広告（こうこく）が印象的（いんしょうてき）だからにほかならぬ。
The reason why our products are popular is simply because our advertisements leave a lasting impression.