https://en.oxforddictionaries.com/definition/them
A couple of other examples: 1. https://www.youtube.com/watch?v=GB33z9xoSw8&feature=youtu.be... 2. https://www.youtube.com/watch?v=elUsB4y5RVk
There are plenty more examples on Google.
--
I want to say I take your point. I wouldn't use this in formal writing, but as far as I can tell this is an accepted western way to emphasise something.
No, it isn't. Using "them" in the way you did makes you sound like a hillbilly.
Just some advice: it's wise to avoid using any words or usage listed in the dictionary as "dialect" unless you're very familiar with the particular dialect it's from. Likewise, you should be very careful with "informal" terms unless you've actually been introduced to them informally by someone who knows how they're used. There's a lot of cultural context that needs to be mastered with those, and you won't learn that from a book, let alone a dictionary.
*edit - I will add that the 2 youtube videos are not good examples to be learning english from...
In your original sentence, you are trying to use "them" as an adjective to modify "incompetent developers," but "them" should only be used as a 3rd person plural pronoun. Using "them" as an adjective is not proper; your examples sound strange to native speakers, and is usually associated with vernacular English in general. Native speakers may assume this usage is due to poor knowledge of proper English, or possibly a regional (improper, but common) usage in the American South.
https://grammarstars.blogspot.com/2008/08/41-them-and-those-...