Sure there are 5 World Trees - technically. Teldrassil was made later than the rest by Fandral Staghelm with the explicit purpose to regain their immortality and have it corrupted by the Old Gods - it was only recently cleansed and blessed. The other four World Trees have been corrupted, destroyed, or weakened. One only exists within the Emerald Dream's version of Un'goro, and doesn't even have a name.
More connected to the Old Gods were the Great Trees, which there are also 5 (Ashenvale, Feralas, Duskwood, Hinterlands, and Crystalsong). Those were created to negate the power of Saronite popping up all over Azeroth, and act as gateways to the Emerald Dream from the physical world. However, they acted as a catalyst for Yogg-Saron to first create the Nightmare, instead.
Though, in the end, I don't really see this theory holding up much. I can see why people like the idea - a burning Teldrassil is a literal torch in that regard. However, it doesn't really make much sense when Vordrassil, Teldrassil, and the Great Trees were made by Fandral to suppress the Old Gods or act as a conduit for the Old Gods' corruption in the first place. Only Nordrassil was explicitly made with the cooperation of three Dragon Aspects to bless it for the Night Elf people. We don't even know about the origins of Shaladrassil (other than it existed before the rest, before the Sundering, according to Oakheart's lore), and we have no idea about the Un'goro World Tree other than Malfurion stated it is a World Tree.
It's a somewhat broad, ethereal topic to use as the catalyst to progress the Old God/N'Zoth plot - two of the five World Trees were introduced (and barely given lore) last expansion, and the rest were given more lore right before the expansion in Chronicle. The Pillars of Creation, while also only appearing last expansion, were given a decent amount of lore, and fit perfectly into the story at the time. We also know that N'Zoth was involved in the area that would become the Broken Shore/Tomb of Sargeras, and that there was something always of interest there. It makes more sense that by using the Pillars in a place that N'Zoth had some influence over would somehow unlock his prison, or something.
The other theory is that the quote wasn't speaking about the Old Gods/Void, and was instead talking about the Burning Legion and Argus. The Pillars were used as keys to unlock the Tomb of Sargeras, which in turn took us to Kil'jaeden, which in turn Illidan opened the way to Argus. That, in turn, led (eventually) to Sargeras stabbing Azeroth, which the Old Gods and their minions are clearly taking advantage of. Immediately, the silithids were empowered by Azerite in Silithus. The Twilight's Hammer stayed active, with new prophecies. And, clearly, N'Zoth, G'huun, Azshara, and the Aqir become more active next expansion, using the faction war and the discovery of Azerite to their own advantage.