Why would topolects get described in terms of Mandarin? (声母变异、韵母变异) [Perhaps this is a "which came first - the chicken or the egg?"-type question - but I'm left quite puzzled why anyone would describe things in this manner.]

Why would anyone (or any material for that matter) describe dialects/topolects in terms of Mandarin? Most topolects/dialects would certainly be much older than MSM (Modern Standard Mandarin) - as a, relatively, new 'invented' language.

e.g.: 广安方言与民俗词典 describes a bunch of 广安方言 in terms of: 


  声母变异


and


  韵母变异


even going as far to describe some words as:


  自创音节


here's some examples:

声母变异: “敲” kao1 - “qiao1”的音变。指打，击。

韵母变异: “眉” mi2 - 眉“mei2”的音变。如“眉毛”、“峨眉豆”等。

自创音节: “□” pie4 - "指破烂，坏，不好。常写成“孬”。

I highly doubt that these are "变异" seeing as these pronunciations outdate the modern "standard" ones - so why would anyone go to the trouble of describing it this way? besides the fact that MSM is the, now, "standard"?
Yes or No.

In Chinese linguistic study, there are three rules of distinguishing dialects from languages:


Linguistically, mutual intelligibility
Historically, derivation
Politically, anything except the standard


Dialects are picked out mainly because MSM gets politically recognized as a standard in China Mainland.

In practice, a minor reason is that they do not have matured or accurate writing system.