While true, Sanahtlig makes guides on the idea that people DON'T need to spend 16+ hours a day on KH like you and I do.
And yet, people who input far, far less effort than us can clear stuff just fine. Are they going to be slightly slower without elemental advantage? Yeah, probably. But there are advantages to sticking to a single element. Anyone can build a Grid for every element but having a good team for every element is an entirely different story. A 140% Grid and a team of SRs and maybe a couple of random SSRs with elemental advantage probably won't hold a candle against a 140% Grid and a team of key SSRs from the element you spent 2-4 Miracle Tickets on, even if the Miracle Ticket team doesn't have elemental advantage.
I don't think Sanathlig has claimed that people who spend far too much time on KH are bad in any way - he simply doesn't see the value in spending so much time for grids that'll never be able support the kinds of teams one would hope from that element.Now, my Fire team certainly isn't good - just 3 random SSRs basically - I picked a Rag Disaster and duo'd it with random host. I picked Belial, so I thought I'd do pretty good but... I got demolished by the Water team the host was playing. I certainly didn't expect that.
Although that's not solo, it certainly should be doable. And once the other Ragnaroks come along, they'll probably (maybe? hopefully?) be even more off-element friendly due to debuffs actually being usable.