Wiktionary is primarily a word dictionary, and as such, some of its phrasing around glyph origins (which is a character-oriented study) may not be clear. The same unclear description issues appear in numerous other characters, like Glyph origin of 「四」, where it is implied that 「亖」 evolved into 「四」 (which is not true, as the 2 characters are unrelated). I would rephrase the section like this:
Here, we clarify that the character 「蝠」 did not evolve from a pictogram of a bat (we'll use 「🦇」 from now on); no component of 「蝠」 is related to 「🦇」.
To answer the queries:
In a case like this, are the pictogram and phono-semantic compound completely unrelated in shape?
In this case, 「蝠」 and 「🦇」 are completely unrelated in shape. We don't really even know what word 「🦇」 is supposed to represent, as it's used sparsely in the late Shāng period as a proper noun on ritual vessels.
Generally speaking, you can only determine the pronunciation of a pictogram (and thus determine the actual word it represents) if it appears in (1) continuous transmission of texts throughout historical periods of China or (2) poetry rhymes. 「🦇」 has neither historical continuity nor appearances in a poem, with 「🦇」 appearing in the Shāng period and then abruptly falling out of use.
This is not to say that pictograms do not have related phono-semantic compounds, or that those that don't have any relation can't be pronounced:
Is 畐 strictly phonetic? I found an (unsourced) blog post that suggests the 扁 and 畐 in 蝙蝠 have meanings of 'flat' and 'cling to', respectively (conveying the meaning of 'animal that clings to flat walls').
Since they don't reveal sources or reveal how they came up with this, the default position is "yes, 「扁」 and 「畐」 are purely phonetic".
It pays to go through the same motions as scientific methodology with such a claim. I don't know where 「畐」 suggesting the meaning cling to comes from, but 「蝙」 related to 「扁」 (meaning flat and thin) is worth investigating. The available data used to test such claims is in the form of variant spellings.
At this point, they should do a search for variant spellings of the word through historical literature:
- Has 「蝙蝠」 ever been spelt with 「扁」? If they can't reference any, then you should reject the alternative hypothesis, that 「扁」 is semantic in 「蝙」;
- One of the alternative words for bat 「蝙蝠」 (Zhengzhang OC: /*peːn pɯɡ/) is 「服翼」 (/*bɯɡ lɯɡ/), indicating that /*pɯɡ/ is a morpheme for the meaning bat. By itself, this casts doubt that 「畐」 has a strong semantic link to bat. Can they then find a concrete word reconstructed as /*bɯɡ/ or /*pɯɡ/ meaning to cling, spelt as 「畐」 or a derivative character? If they can't, then you should reject the alternative hypothesis, that 「畐」 is semantic in 「蝠」.