grammar - Plural / Singular verb in this sentence?

Recently an editor just changed the plural verbs in my essay to singular ones, but I don't understand why he did so. Please see below:

(Original sentence)

After rising to power through multiple revolutions, the Chinese Communist Party holds a subversive notion and ideology that not only aspire to break the old world, but also aim to build a new one.

(revised sentence)
After rising to power through multiple revolutions, the Chinese Communist Party holds a subversive notion and ideology that not only aspires to break the old world, but also aims to build a new one.

In my understanding, there are two nouns in "a subversive notion and ideology", and that should be followed by a plural verb?

Answer

The key is in the use of the indefinite article.

To paraphrase:

The CPP holds a ^{subversive notion and ideology} that aspires . . .

I have put the subject in superscript in order to set it off from the rest of the text. Especially because of the a, the syntax treats the phrase as a compound single subject.

It is the same idea as:

This ^{fish and chips} is delicious.

Although there are actually two separate things joined together, they are thought of, and treated as, a singular item.

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In order for the syntax to show that there are multiple things being used as individual subjects, you would need to remove the indefinite article and pluralize the subjects:

The CPP holds subversive notions and ideologies that aspire . . .

Alternatively, it's possible to use a phrasing similar to what follows—but it would be somewhat awkward:

The CPP holds a subversive notion as well as a subversive ideology that both aspire . . .