I'd say by "expert" they are familiar with the basic interfaces used on many operating systems. Do they know how to create a word document without hand holding? More than likely. Can they create a basic spreadsheet? Probably. Do they understand how to use office (MS or open or whatever version you pick) to its fullest? No. My experience is that many millennials seem to think "expert" knowledge of such software suites comes easy and they actually have it but get frustrated quickly when asked to do something complicated (like db links, mutli-sheet vlookup or *gasp* vb macros ).
Many (most?) of these so-called "expert" Millenials don't know shit. All they know is how to click on FB. Fortunately there are a few who have actually grokked IT, and those are the ones to employ. Unfortunately, most PHBs don't know this...