The implementation tries to be as compatible as possible. Apart from that, it's implemented using a language implementation framework (rather than from scratch), and written in Java (rather than in Smalltalk like the state-of-the-art OpenSmalltalkVM, or some low-level language).
> Is it that you can easily call to Java and JavaScript (and perhaps back)? > In other words, Why TruffleSqueak?
We are using TruffleSqueak as a research platform for polyglot programming. But, of course, it can do a lot more. The Smalltalk programming system runs on the same level as all other languages and on top of GraalVM. This allows direct interaction from Smalltalk tools with the GraalVM runtime. In the "Live plots with ggplot2" demo [1], for example, we briefly show a visualization of the Graal compilation queue. Usually, you'd have to go through something like JVMTI, but TruffleSqueak has direct access to the runtime.
I guess an alternative answer to your question is "why not?" :)
[1] https://twitter.com/fniephaus/status/1264839969115340800