Look at Step 6...

It look like if you take off that metal plate, there's room for a 12mm or 15mm hard drive, if you also don't use the caddy. Just don't use that (seemingly useless) metal plate (or better yet, just trip off the area that would cover the hard drive).

So maybe 9.5mm HDD (currently, 2TB) is the limit for those who are not modders and respect warranty. But that's not those of us on this site!