They were still likely benefiting to be fair. Remember stamp duty is paid externally to the mortgage so it effectively comes off your deposit, not your loan.
Because the author doesn't want to intimidate new users by making them feel this is necessary knowledge to learn react. As someone presumably more knowledgeable in the field, it's much easier to judge whether an article is at the right level for you than someone just starting out.
What are the cost implications for using this (in terms of the backend maps providers)? I looked at MapBox's pricing, for example. Does that still apply when using this? Presumably so but I find the distinctions a bit confusing in the JS maps world.
No, not in this case. It's running in JS land in the browser so any exploits to break out of the browser sandbox using this could be done simpler with raw JS.