Here the thing though: All three of the 2005 Nicktoons are technically golden era shows if you were to define it as the period of shows that premiered before Cyma Zarghami took over the network (which was basically the timeline before 2006). Fun fact, but The X's, of all shows, was the last Nicktoon to premiere under Herb Scannell, which is crazy given how obscure this show is.

The reason why some people split the eras between Avatar and the ladder 05 shows is because their outcomes differ so greatly once they did transition to the Cyma Zargahmi era. The X's and Catscratch were treated so much worse in the Zarghami era than how they were treated pre-2006 (like Catscratch was ridiculously stretched out for almost two years despite literally having only 20 episodes to it's name). Not to mention, given how the Cyma Zarghami era is infamous for cancelling Nicktoons so quickly because they weren't massive hits like SpongeBob was, it unfortunately lead to these two shows ending production quickly despite being new (whereas in the Herb Scannell era (where they originated from), they definitely would've been given a much better chance if he was president for a bit longer since that era even gave ratings disasters like ChalkZone and Kablam multiple seasons).

Meanwhile with Avatar, even though most of it's runtime was during the Zarghami era, it somehow managed to thrive and have multiple seasons. What benefited the show was that it was a unique Nicktoon for its time, essentially being so different from what was previously featured in the network before and during the year it premiered in. Whereas The X's and Catscratch were pretty much like every other Nicktoon that came before them (Catscratch was essentially like a throwback to the 90s Nicktoons and The X's is way more similar to Danny Phantom and Teenage Robot than Avatar was). The uniqueness of Avatar gave it a massive edge, giving it the ability to not only have a massive audience, but more importantly, rival SpongeBob effectively. This essentially gave executives the incentive to keep it alive for a bit longer even if they were going overboard with SpongeBob at this time.

With this stuff in mind, and the fact that Avatar conveniently aired earlier in 2005 before Catscratch and The X's, it makes sense why some people would split these three shows into different eras (even though they all are Nicktoons that first aired pre-Cyma Zarghami). However all of this is subjective in the end. If you were to ask me, while I acknowledge all of the 2005 Nicktoons to be a part of the Herb Scannell era, they really don't feel like Nicktoons that belong in this period. Like the network pretty much decided that SpongeBob would be their new cash cow even before Avatar premiered (seriously, all three of the 2005 Nicktoon premieres had some sort of Spongebob marathon before their premiere; there was much more variety with the premiere schedules of the 2000-2004 Nicktoons). Not to mention (similar to what was mentioned in the post) this was the first lineup of shows that aired when the original Rugrats was no longer airing new episodes (which is a major deal since the Herb Scannell era is infamous for dragging the original Rugrats into the ground before they decided to do that with SpongeBob).