Why is that so unbelievable? TypeScript isn't JavaScript, and while they have the same runtime, compiled TypeScript often don't look like how you'd solve the same problem in vanilla JS, where you'd leverage the dynamic typing rather than trying to work around it.

See this example as one demonstration: https://www.typescriptlang.org/play/?q=8#example/enums

The TS code looks very different from the JS code (which obviously is the point), but given that, not hard to imagine they have different runtime characteristics, especially for people who don't understand the inside and outs of JavaScript itself, and have only learned TypeScript.