Hey there. One thing to consider about most guides on forum is they are designed with high end-game raiding in mind, so, their argument might not always hold valuable for gearing/learning.
If you know how to use Intellect as Disc Priest, it would be the most valuable stat for Mana regen, then you can invest on other useful stat like Haste, but while gearing up and learning, Spirit helps you to have a safer Mana return in case of spam or mess ups + while gearing you probably doing a lot of 10 man raids which means less access to Mana regen stuff like GBoW, Replenishment, Innervate and etc. which makes spirit more shiny and useful, but in 25s and high-end raids with organized guilds it loses it's value a lot.
So in the end you are better off with spirit items if new to spec, just have an eye on your haste level (the guide you linked explained it for example) if you want to really min-max + you will change your gear soon enough, don't over think it.