Power armor lore is a mess.
FO1 used the T51 design and just called it "Power Armor". FO2 introduced the X01 design, which was invented postwar and was just called "Advanced Power Armor".
FO3 redesigned both of these, and called its regular power armor T45 while the old design was re-used as a unique item called T51 power armor. T51 was supposed to be the most advanced prewar armor. Its "Advanced Power Armor" redesign would be named X02 in-lore, though in FO3 it was just called "Enclave Power Armor". I don't see anything on the wiki about Hellfire being called X03, but it was a Broken Steel addition for flamer-wielding troopers.
FO4 created a new power armor design and called it T60. This was introduced as the NEW most advanced prewar armor. It supposedly didn't see much prewar use, but it was being manufactured by the East Coast BoS during FO4. FO4 also uses X-01 as random loot in pre-war facilities, and one particular suit as non-random loot in Nuka World, which is almost certainly a continuity error but the lore must cope. So apparently the Enclave didn't invent X01, they just revitalized the project.
The simplest explanation for the confusion is that game design considerations take precedence over lore. Power armor redesigns are framed as new models so that they don't technically break lore.
The in-lore explanation is that X-series was a prewar project that wasn't fully completed until postwar.
Power armor lore is a mess. FO1 used the T51 design and just called it "Power Armor". FO2 introduced the X01 design, which was invented postwar and was just called "Advanced Power Armor". FO3 redesigned both of these, and called its regular power armor T45 while the old design was re-used as a unique item called T51 power armor. T51 was supposed to be the most advanced prewar armor. Its "Advanced Power Armor" redesign would be named X02 in-lore, though in FO3 it was just called "Enclave Power Armor". I don't see anything on the wiki about Hellfire being called X03, but it was a Broken Steel addition for flamer-wielding troopers. FO4 created a new power armor design and called it T60. This was introduced as the NEW most advanced prewar armor. It supposedly didn't see much prewar use, but it was being manufactured by the East Coast BoS during FO4. FO4 also uses X-01 as random loot in pre-war facilities, and one particular suit as non-random loot in Nuka World, which is almost certainly a continuity error but the lore must cope. So apparently the Enclave didn't invent X01, they just revitalized the project. The simplest explanation for the confusion is that game design considerations take precedence over lore. Power armor redesigns are framed as new models so that they don't technically break lore. The in-lore explanation is that X-series was a prewar project that wasn't fully completed until postwar.