It isn’t possible to prevent any app from being decompiled, even in Android Studio, all code can be decompiled. The discussion here is whether Thunkable use ProGuard to obfuscate the apk. This jumbles up the code and makes it almost unreadable to help stop the code from being copied and to protect sensitive information from being accessed.
I decompiled an app created with Thunkable X and it looks like the code is obfuscated, most of the classes, packages, variable names and methods have randomised names.