My opinion is that <indeed> would technically require you to include <en effecto> or something like that, and by doing so, you would ramp up the emphasis to an even higher degree. While the emphasis might be slightly greater with <do indeed>, the meaning would be essentially the same.
But, why? It's almost like a double positive, similar to a double negative in English. Gramatically (at least in English), it's deemed to be incorrect grammar. If the statement without "no" is already affirmative/positive, why put the extra "sí" unless it's to add emphasis? Just trying to understand the rules.
Is it possible this is a statement that would be negating a previous thought or to answer a question?
¿Los atletas comen arroz? (Do the athletes eat rice?)
Me pregunto si los atletas comen arroz. (I wonder if the athletes eat rice).
Answer: Las atletas sí comen arroz.
Thoughts? I am just trying to understand why I would use this sentence structure.
Here the issue is not about what is satisfactory, what is good and what is better. The point of inserting "sí" in the way that it appears in the sentence is to teach something new. If you simply revert back to "sí" means "yes" and I'll rearrange the words to make that sound correct in English, you'll have missed the opportunity to learn this idiom.
The English translation is correct with or without the definite article. The fact that the Spanish has the definite article means the speaker/writer is either referring to a specific group of athletes (which seems the most sensible usage) or speaking of all athletes in general. It could have been omitted altogether, but that would mean something different. Without a definite article, it would be about some undefined or otherwise nonspecific group of athletes, but not athletes in general.