grammarly uses statistics to enhance their models, I'm pretty sure.
The idiom without article can be found frequently enough that it might distinguish certain uses following a basic grammatical model.
I guess the participle throws it off because indeed it sounds odd. And they might support this finding with syntactic models.
Abstract nouns, often characterized by adjectives, take an article; "modern society" does not, because it's treated as a compound noun. The participle does count as (a) adjective, therefore an article is needed. This is just a rule of thumb I think, but it's reasonable to mark the head of a longer noun phrase clearly as such. It helps parsing. (b) technically it is a participle phrase, but for sake of illustration consider in a green and evergrowing society. Removing the article does not work here either.
Also, you need two commas if you omit the article, so that you have two sentence adverbial phrases, basically as if people or have was characterized by .... But, that is probably not what you intended. The same could be however an unimportant paranthetical. It is not.
The characterization is vital, then, because you select one of a many different modern societies.