You appear to be asking two things.
1) Why 知られる is used instead of 知る in your example.
2) How often 〜ことから〜される and 〜ことから〜できる are used.
When you say "potential form", for 知る, I think you may be confusing "知られる", the passive form of 知る, for "知れる", the potential form of 知る (though many people, including myself, would say 知ることができる instead of 知れる).
In your final example with これらのことは、…話し方から知られることになるだろう, the topic of the sentence is "これらのこと" (these things). By using the passive voice, we don't need to declare who/what the do-er of the action is. (See this post for a deeper explanation.)
These things will most likely be picked up / become known not so much from what the other person says, but rather from the way they say it.
We do use 〜ことから、〜される and 〜ことから、〜できる frequently, but you're more likely to see it in 文語 or hear it in an academic presentation / perhaps on the news (I can't remember ever consciously hearing it on the news, but it seems like something they would say.)
名詞+から has the exact same meaning/usage as adj./verb/adverbial(連体形)+ことから in the link broccoli forest shared.
As to why one would use it in the potential vs the passive voice, in the passive voice, the speaker sounds as if they're distancing themselves from the subject. Maybe they haven't thought it through completely or disagree with the premise. With the potential form, however, the speaker is showing his agreement with the premise that is presented.
It is said that... (I'm not saying whether I agree or not)
It can be said that.... (I agree with the premise)
In response to E. Matsunagaさん's comment regarding 言う, I've 追記した'd the following 補足説明.
Let’s continue with your 祖とも言われる / える example.
With 言う, just looking at the sentence as is with no further context, there appears to be an implied subject such as (話し手の共同体を表す)我々, which not only implies the speaker’s agreement with the premise, but also that of a larger group of people the speaker is a part of (such as a company or research team, or even just the world-at-large). If there were to be a explicit subject such as 私, that would imply that the speaker definitely agrees, but not necessarily that anyone else does.
This table might help make it easier to understand:
(Note that without an explicit subject, 言う is most often going to be referring to a large group of people or the world-at-large saying something)