based on several references, main thread is different with UI thread, but UI thread can only be touched when we are on the main thread. so when we're talking about UI I think it's legit to call it as MainThread. But remember, there are some situations where MainThread work without UI thread
https://medium.com/@nebp314/android-is-main-thread-same-as-ui-thread-4ecb30c8bf42
https://developer.android.com/guide/components/processes-and-threads.html#Threads