So Swift didn't turned out like they imagined and Rust is just the next best alternative to that failed vision using Swift.

So far this is the first and only shift

They were doing their own custom language before Swift.

didn't know

> The browser and libraries are all written in C++. (While our own memory-safe Jakt language is in heavy development, it’s not yet ready for use in Ladybird.)

https://awesomekling.github.io/Ladybird-a-new-cross-platform...

only thing I could find - has it been actually used in Ladybird after all?

From the link it seems that Ladybird architecture is very modular, in this case LibJS is one of the subsystems that has less external dependencies, said that they don't need to migrate everything, only the parts that makes sense.

Yes, i understand that in a personal project, but they have investors behind them.

They adopted Rust for LibJS, not the browser and its engine.