Also back then people traveled less (especially in the lower classes) which probably made accents stronger and more easily identifiable. Now it's routine for people of all classes to move to a different part of the country for studies or work, and you have mass media spamming a somewhat "standard" Parisian French across the country.
And speech is still the main interface of communication. In general when people send casual texts they'll try to emulate the spoken language, including nonstandard inflections and spelling changes etc...
If anything on average we probably pay a lot less attention to the written word than we used to because we use it so much more and for much more casual conversation. Few people used to write "wanna grab sumthin 2 eat?" a few decades ago.
So, while what you said may be true for radio broadcasts, I am not sure it is at play here even if the interviewee knows he is being recorded.
Someone from 1910 would have a hard time understanding all the neologisms that were introduced after two world wars and decolonization until today.
For us from 2021, someone from 1912 uses words that are commonly found in written works from the same period (and after). This is why we feel it sounds like 'art'. Written words live way longer than spoken words.
Even today, common spoken French is very different from common written French.
I mean one of my favorite channels on youtube is a pair of disembodied Canadian hands and voice, he has a way with words: https://youtu.be/toewD0VInlc. Here's a dictionary (do set it to show more than 10 at a time): https://codepen.io/LegoLife/full/YVXbMR
Although it might sound clearer to English speakers because it features more word-level stress while standard French doesn't really have it (but this has not been lost recently).
Similarly I've been learning Spanish the past few years, and the most comprehensible accents are those from Guatemala, Nicaragua, and Honduras--some of the poorest Spanish-speaking countries. I cannot for the life of me understand a heavy Mexican or Chilean accent, but I can easily follow Guatemalan. It's interesting from a socio-linguistic perspective if nothing else.
This is still the case today but was even more so and educated upper classes people were taught to speak 'properly'. That very posh accent you hear in old British films and old BBC footage is called "traditional received pronunciation(RP)" [1].
That being said, accent is a social marker almost everywhere. It certainly is one in France, including because the country is very centralised on Paris and regional accents are usually deemed 'inferior' (it's similar to England, tbh). These days in France the accent and way of speaking you want to avoid at all cost is "l'accent des cités", i.e. the accent of people, often of foreign descent, from the bad suburban areas.