Q:

Why does は refer to a particular rather than general in some cases?

The sentence I am asking about is the following

[魚]｛さかな｝が[好]｛す｝きじゃない[人]｛ひと｝は、[肉]｛にく｝が[好]｛す｝きだ 」
Person who does not like fish like meat
Source: Tae Kim's Guide to Learning Japanese

My understanding, which is probably flawed in some aspect, is that は after 「[魚]｛さかな｝が[好]｛す｝きじゃない[人]｛ひと｝」 should make the sentence mean "People (in general) who do not like fish like meat", since は would mean in general as opposed to a particular occurrence (が).
How, then, would the general form be conveyed (i.e. "People (in general) who do not like fish like meat")?

A:

You are correct and that website is incorrect on this matter.
Upon hearing/reading the sentence:

「​魚
{さかな}
​が​好
{す}
​きじゃない​人
{ひと}
​は、​肉
{にく}
​が​好
{す}
​きだ。 」

Practically all Japanese-speakers will take the 「人」 to mean "people in general".
It is just extremely unnatural to form that sentence when the speaker/writer is referring to one particular person.
To alter the sentence so it talks about a particular individual, one could say:

「魚が好きじゃない〇〇さんは、肉が好きだ。」 or more naturally,
「〇〇さんは、魚は好きじゃないけど、肉は好きだ。」←Uses a pair of the contrastive 「は's」.

