I think the purpose of this post was to show that there really isn't a huge difference early on in Mono vs Rainbow grid. So, I think it's to help players that are starting/still building.

He even mentions in the beginning that the endgame is to get to a Mono SSR Grid. He is just showing a little bit of difference in how ppl can get there.

With the introduction of daily Gem caves it can now be a little harder to argue which ones builds faster (although do newer players really have enough HE to hit the gem caves that hard to build the mono grid faster?). Rainbow Grid is easier because you only have to SL the SSRs, and get a higher Atk and HP Quickly. Whereas the mono grid you have to make sure you get the right weapons, and spamming Disaster Raids takes a while to get all the weapons you need. Then Skill Leveling takes a lot of time and resources.

Players can choose whichever way they please to build up to that grid, and this was just intended to show that going the Rainbow route initially may not be that bad. Personally I have decided that I will skill level each of my grids as that Ele comes up in a Union Raid. So, since the end of last Union raid I have been working on my Wind Grid, and am using a Rainbow Grid for this event. I have decent grids for Fire, Water, and Light. My Rainbow grids have been working just fine for the other 3 elements. However I'm almost at a 50k Total Atk rating with my Rainbow grid.