You contradict yourself.
1. "If the difficulty is out of the lower range, your success chance of making the item is 100%, so you don't have the chance of skill gain."
This is a direct contradiction of your earlier post.
When the Talisman is applied to your skill, it can take you out of focus area, making you incapable of gaining skill. Therefore, Talismans DO affect skill gain (by blocking it, if worn when the skill is too close to the top end of the focus). And, this is the situation where "can I gain?" questions will come up. Trying to argue otherwise is playing games of semantics. But, that's just the tip of the iceberg.
2. There are several other items that are questionable.
a. Difficulty. Difficulty has a basis in skill, but its basis is indirect - the actual chance to gain is based on the total percentage chance of success, not the one solely from skill. The way Talismans were described in the same Dev post and follow-ups, indicated that the talisman bonus is applied to the success chance in such a way that the chance to gain treats the modified difficulty, not the unmodified one, as the basis for gain chance. Once you have a percentage chance to make, the skill value is irrelevant.
b. Furthermore, The devs have also stated, and years of testing and skill gain charts have borne this out, that powerscrolls alter the Focus range for blacksmithy & tailoring (and have a similar effect for non-crafting skills). And, they alter it in a way that shows the gain is based on success percentage, not the skill number directly.
For these two crafting skills:
If you use a 105 Powerscroll, your focus is expanded to where you can gain on an item where its hypothetical chance for success is 105%, instead of 100%
If you use a 110 Powerscroll, your focus is expanded to where you can gain on an item where its hypothetical chance for success is 110%, instead of 100%
If you use a 115 Powerscroll, your focus is expanded to where you can gain on an item where its hypothetical chance for success is 115%, instead of 100%
If you use a 120 Powerscroll, your focus is expanded to where you can gain on an item where its hypothetical chance for success is 120%, instead of 100%
In these most-obvious cases, the actual "Focus" in terms of skill, has the end increased by half the percentage, or 2.5 points per 5%. The question now becomes, does the ratio increase to be "x/(cap for skill)" losing the ability to easily define the change in tenths of points of skills & percentages; or does it just shift the midpoint 2.5% per 5% of scrolled change to the skill cap (where, on a 105 scroll, 52.5% becomes the new midpoint, etc.)
c. It also further breaks down in that, while some skills have the ability to make items start at some number other than 0%. Specifically, how is Focus defined for Tailoring, if most items start at 50% at the point where one can first make them? Is the focus (without power scrolls) still 50, with skill gain started at the optimum point? Or is it 25 (the distance between the point where the skill allows the making of most items, and where that item hits 100%)?
While the "Focus" was defined in terms of skill by the dev in question, it is apparent from these facts that he was doing so as a means to simplify the true mechanics that underlies the skill gain apparatus to be more understandable, while sacrificing some preciseness - that of the difference between 0% chance and the capped level of the skill, which serves as the opposite end of the gain spectrum. The "Focus" as described in skill is a surface effect of a deeper underlying mechanic grounded in the skill percentages. The effect, if you will, not the cause - and it is easiest to explain for a non-scrolled skill.
And, BTW, I couldn't get a Guaranteed Gain in 20 successes with a talisman on, with an item that would have gotten me the gain without the talisman equipped. That's good enough evidence for me that talismans effect the skill gain root numbers on the skill "modified percentage to succeed" level, not the skill level directly.