"the same sound as "ьо"
No. There are separate, while in practice related, effects to be considered there:
short pause after the consonant before the vowel. The separation between two sounds. Prevention of blending.
softening of the consonant
softening of the vowel ( back "hard" vowels like
Ы are matching front "soft" vowels like
prefixing the "soft" vowel sound with short "й" sound (iotation)
So, "hard mark" and "soft mark" (Ъ and Ь) primary purpose is #1 - making a pause.
Then, the "soft mark" also softens the consonant, unlike the "hard mark", but this does not remove the pause.
Now, the vowel pairs а/я, о/ё, э/е, ы/и, у/ю while primary are representing #3 typically have a consequences of effecting either #2 or #4.
When a "soft" vowel stands on its own or follows another vowel, then ё would sound more like йo. When following a consonant, the soft vowel blends in, softening the consonant, instead of prefixing its own sound with "й".
However both options #2 and #4 do NOT add the separating pause (#1), which is the hallmark of hard/soft marks.
All in all, почтальон is not pronounced as почталён, but as a почталь-pause-ён.
This pause makes putting the soft mark required.
Now, you may ask why isn't
почтальОн then written as
почтальЁн instead. Well, soft vowels are not typically written after soft sign. Hard vowel is written there traditionally, and it inherits its softness from the now softened consonant before it (despite being separated by a pause).
To contrast it,
кЛЁН are sounded with no gap and no
й, but immediate blending of the vowel into a softened consonant
почтаЛЬОН is sounded with the gap, and because of the gap - with
й sound. Yeah, it is mixed up a bit:
- on one hand, the process of softening the consonant is "jumping over" the gap and softens "o" vowel into de facto "ё" (in sounding, not in writing!).
- on another hand, the gap makes the vowel "standing on its own" which forces й-prefixing, like if the vowel would start the word or would follow another vowel.
Indeed, rather convoluted scheme. Gap is ignored for one process but is abided by for another.
It would stress the tongue a real lot to blend hard consonant sound with following soft vowel sound, or soft consonant with a hard vowel. It just does not work out without injecting a separating pause (which in turn would effect soft vowels to regain their Й sound prefix).
Pronunciation samples of syllables with separating pause and with blending..
"Кто там? Почтальон Печкин"
"Старый клён, старый клён, старый клён стучит в окно"