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Even the examples provided are kind of Math-heavy (for example, jumping right into the notation of the limit of x^5/25) - enough that a novice would probably feel overwhelmed.

I find that people who are very familiar with a subject usually have a difficult time explaining it without slipping into jargon that will lose the listener.

Is there a word for this effect in education? Where the teacher forgets the context of the learner?

"The curse of Knowledge"

https://en.wikipedia.org/wiki/Curse_of_knowledge

Also Shoshin, "beginner's mind":

https://en.wikipedia.org/wiki/Shoshin

Expert blindness.

Assuming a learner of calculus has already learned basic algebra?

You're right. But not for this statement.

At most, he could have written "The slope of a linear equation or "line" is constant. This is a known fact."