I've heard this from people who like the new Summoner all expac. It's just not true. What you've listed are just things that define the caster role; positioning and cast times. The difference being here that Red Mage and Black Mage have to actively adjust their rotations and what they're doing on a per GCD basis; Summoner gets its on-rails rotation reset every 60s and if you have to drop your Ifrit dash for a Ruin III, the potency lost is so minor in the grand scheme of things, it doesn't matter. There's no fail state for messing up Summoner, you're rewarded for existing by having virtually limitless free movement and damage where RDM and BLM have to work for uptime and movement, especially in this tier.
Once my static cleared P7S back in September I eventually opted to just do reclears on Summoner over Black Mage simply because Summoner is competitive and puts no work in versus the amount of struggling RDM and BLM have to do, especially with mechanics like Purgation that require you to pool all of your resources to keep full uptime and drain you dry only to dump you into Harvests immediately after. Boss hitboxes are so huge this expac that you don't have to think about "is the boss going to stand still long enough to get all the ticks in" (I would use Slipstream on Jumps 1 in P8S and Hephaistos' hitbox is so dummy thicc that he'd be in Slipstream still when he moved back to the center) and 3/5 bosses this tier literally do not move (P6S, P7S, P8Sp2). What you're saying doesn't reflect the game at the point it's in right now, and that's the problem.
Summoner is a cancer for balance within the caster role. It cannot continue to exist in this state while doing more damage than RDM, having nearly similar utility, having better movement and having no fail state beyond dying which is the fail state for every job in the game; your entire fight timeline is not messed up in the way that it is for RDM and BLM because it resets every 60s. Either make Summoner a physical ranged job and balance it according to the ranged DPS, make it an actual caster again, or make it worse than RDM because it has no reason to be doing similar or better numbers than a job that has no personal mitigation and is working much harder to do comparable damage. This tier especially has shown that the ability to raise is not as useful as it once was, meaning RDM has no genuine advantages over Summoner, especially not to justify it doing worse damage.
I don't even hate the concept and what they were going for on Summoner, and I think Astral Flow could potentially be one of the better job mechanics they've ever come up with. I just sincerely hate the execution and how it has destroyed the entire caster role for this expac.