Sadly, it's not. I can't tell you the number of 5-8 years' experienced candidates from respectable companies who couldn't solve very easy interview problems.

Yes, I get that a whiteboard isn't an editor, and you don't have google/SO, and some people just freeze in an interview, but I've long ago stopped assuming people can code just because they've got coding on their resume for successful companies. (For all I know at the interview moment, they didn't have a coding role, or may not have even worked there.)