This is fatuous bullshit. Yes, it CAN happen. And I could win the lottery -- which is more likely than an LFR player outgearing a mythic raider, when looking at total ilvl in all slots.
On average, a raider on a given difficulty level will get better gear, and get it sooner, than raiders on lower difficulty levels. The rare (in some cases, astronomically rare) exceptions do not alter this.