Well, then it's a little unclear to me what you are arguing against. That they aren't from "roughly the same time period" from the perspective of someone that isn't familiar with art history? Why would someone unfamiliar with art history make a distinction between early 20th century art movements? They both occurred in the early 20th century, unlike (as I used in my example) neoclassical and medieval styles, which occurred centuries apart.
This is the subject of the grandparent post that I was replying to. Yes, in actuality, they overlapped a bit, and Art Nouveau waned as Art Deco rose – although again, this is not so clear cut in other art fields as it is in architecture; e.g., jewelry or furniture.
WWI was a watershed event. A world-changing event. It wasn't as short as the time period between when the first atomic bomb was dropped and when the second one was dropped, but in the scale of world events, it was still relatively short.
And while Art Deco got started before WWI, it didn't really take off until after WWI.
Just like Art Nouveau that got started in the 1890s, but didn't really take off until after the International Paris Exhibition in 1900.
Both styles took several years to get started and really take off. And Art Deco didn't really take off until after WWI.
This is the subject of the grandparent post that I was replying to. Yes, in actuality, they overlapped a bit, and Art Nouveau waned as Art Deco rose – although again, this is not so clear cut in other art fields as it is in architecture; e.g., jewelry or furniture.