There can be confusions though - the standard way of writing pinyin groups whole words rather than individual characters, so something like "xian" may represent either "西安" (two syllables) or "先" (one syllable).
Or... maybe he's thinking of Chinese opera. Yeah sure when listening to Chinese opera, all kinds of sounds get lengthened into many syllables ;-).
But really I can imagine how this happened. Couple of IPA wonks sit down with a Chinese native speaker, maybe a linguist, and they say, wtf, explain to us what is the difference between 'jun' and 'zhun'? The Chinese person does a hyper-exaggerated slow demonstration of the sounds, and they write that down as the official IPA transliteration that the OP is fixated on, further cementing a misunderstanding of real usage that was implied by the zhuyin transliteration.
As to Xi'an, pinyin has a rule for that kind of case, which is to add an apostrophe in between the parts to resolve the ambiguity.
When I listen to recordings and to people speaking it, it sounds like two, which the Zhuyin seems to support.