(In others, not, you know, cus of the much more recent revolution-ing)
> languages were unified in writing
This is definitely true, and was a major breakthrough
> for speaking, there are only dialects
This is extremely wrong.
The language that my family spoke (taishanese) is not in any way mutually intelligible with the language my wife's family speaks (dongbei dialect of mandarin).
Saying various Chinese languages are all dialects is like saying Geordie English and Italian are two dialects of the same language. Actually even farther than that for some Chinese languages.
There definitely are some languages that are mostly intelligible dialects - all the Wu for example. But then you've got Mandarin vs Cantonese vs Taishanese which is an ascending level of hellish difficulty in understanding (4->6->9 tones, a little different grammar structure, etc).