You're reading this the wrong way I think, citations aren't given because its obviously a pedagogical article about well established stuff. Much like you wouldn't give citations in a blog post explaining calculus.
One could give citations regarding calculus it's pretty interesting. Since it was done twice by both Newton and Liebniz. There must have been cultural developments in the 1660's that demanded calculus be invented.
Yeah I agree, I was just saying I really doubt the author of that blog post was trying to take credit for the ideas they present.
Yes there is chasm between not citing references and stealing ideas. I assume no malice and just want contextual explanations.