I'm not familiar enough with reactive to understand exactly what this is doing but I presume it's ensuring that the throttle call resumes execution on the main thread.  If so that will cause a potential bug in the code later on.  

While an ITextBuffer can be viewed from any thread it can only be modified from the owning thread.  Vast majority of instances are owned by the UI thread but there are instances which are owned by background threads (rare but they do exist).  The Changed event will only be raised from the thread which owns the ITextBuffer.  Resuming on the UI thread means you've potentially moved threads and can lead to an exception on line 136 when you perform the edit. 

All ITextBuffer created on a background thread are not representing user documents and can be safely ignored from this extension.  You can check this by querying CheckEditAccess.  If it's false when you're on the main thread then it's not an interesting buffer and can be ignored