Speed tying with other Choice Scarf Hydreigon's really important and Modest doesn't really improve enough damage rolls significantly to justify being slower than other Choice Scarf Hydreigon.
Yeah, I agree that automatically losing the speed tie against other scarf Hydreigons is bad, but I also feel like it's rare for 2 scarf Hydreigons to face off with neither being choice-locked into something other than Draco. A -2 Draco from Timid Hydreigon also only does 68.9-81.8%, so the Modest one could get a guaranteed revenge kill regardless, if healthy enough. Going for speed ties anyway isn't ideal, but I get that sometimes you need to.
I do feel like some improved rolls do justify considering Modest over Timid. The Corviknight and Excadrill roles with Fire Blast that I mentioned before can be big against the 3rd and 4th most used Pokemon in March. Modest Draco also has an 87.5% chance to OHKO Zeraora after rocks, but Timid is only a 31.3% chance. I feel like that's a significant improvement in the likelihood to revenge kill one of scarf Hydreigon's biggest targets. Modest Dark Pulse also has a chance, although a small one at 12.5%, to OHKO Aegislash-Shield after rocks, while Timid can never OHKO. Corviknight, Excadrill, Zeraora, and Aegislash were 4 of the top 12 most used Pokemon in March, so I feel like improving the damage rolls against them are all relevant.
I understand that Timid scarf Hydreigon offers benefits over Modest and I'm not arguing that it should be replaced. I just feel like Modest offers enough specific benefits with few downsides (mainly Drill after a spin, opposing Timid scarf Hydreigons, and sometimes scarf Rotoms) to deserve being mentioned as an option, especially since the Nasty Plot analysis directly above it lists "Timid / Modest" while citing the improved damage rolls against Clefable. +2 Timid Flash Cannon has a 12.5% chance to OHKO max SpDef Clefable, but +2 Modest Flash Cannon is a 68.8% chance. That's an important roll, but I think that the downsides for Modest Nasty Plot Hydreigon being outrun by unboosted Drill, Kyurem, jolly Mimikyu (matters if busted), and by your logic, other Timid Hydreigons are far worse than the downsides of scarf Hydreigon being modest. I don't see why if the Nasty Plot analysis considers Modest, the scarf set shouldn't as well.