plural noun ends in "-s" as the logical subject of gerund?

I was very amazed by my parents working so hard.

It's example sentence from my grammar book.

I generally understand that the possessive case comes before a gerund—as in “I enjoyed their solving the problem together.”—to indicate the subject of the gerund’s action (in this case, “solve”).

In the sentence I’ve block-quoted above, however, a plural noun precedes the gerund to describe the subject of the gerund’s action (“work”). Also, my grammar book says that since “parents” is plural, I can’t use the possessive case(parents'). Is my grammar book actually explaining this correctly?

I’d like to know if this is grammatically possible. I searched online before asking this question, but I couldn’t find any information on this.

Edit: This question is a little different from the usual ones. Usually, questions ask whether a pronoun like “he,” “she,” or “I” should be in the possessive or accusative case when it precedes a gerund, or they involve common nouns that don’t end in “-s,” such as “girl” or “Jane.” However, my question asks what happens when a common noun ending in “-s”—such as “parents” or “individuals”—precedes a gerund to indicate the agent performing the action.

In the News on the Web corpus, there are 73 hits for about the (plural noun)' (-ing verb), so the version with a genitive plural noun (not a pronoun) is not non-existent.

We both were indignant about the Communists’ thwarting the Olympic protocols against performance-enhancing drugs. (Washington Times)

The survivors and victims' relatives complained about the authorities' letting Amri slip through their fingers, the chaotic response immediately following the attack and what they saw as a lack of financial compensation for their losses. (Brisbane Times)

Last time the question of money came up in the Sunday readings, it was about the apostles’ holding everything in common. (Church Times)

However, there are 15672 hits for the same without the apostrophe after the plural noun, so this construction is quite definitely not the standard. Searching for a similar string with a wildcard for the preposition - (preposition) (plural noun)' (-ing verb) - in the Corpus of Contemporary American English we get 102 hits for the version with the apostrophe and 2000 for the one without.

There is also the issue of whether the non-genitive NP (no apostrophe) is interpreted as the subject of a gerund-participle (-ing clause) or whether the gerund-participle is interpreted a post-head dependent in an NP headed by the plain plural noun. For example,

• We're concerned about the students stealing test papers.

where we could understand "the students who steal the test papers" or "the fact that the students are stealing test papers" / "the possibility that the students will steal test papers" / etc.

But even so, with ratios like the ones above, it's hard to recommend the version with the apostrophe - on the contrary it would probably be best to advise against it unless the writer is 100% confident they need it and know what they're doing.