"But orders are orders."
Translation:Mais les ordres sont les ordres.
"Des ordres sont des ordres" would be "some orders are orders".
The intention of the sentence is that generally orders are orders - all orders not just some.
In such a case when "all" is implied the English sentence leaves out the article. However we can't leave out the article in French.
Depending on the context "les ordres" can mean the specific orders given this morning or it can mean orders in general - all orders.
rester is an intransitive verb. An intransitive verb has two characteristics. First, it is an action verb, expressing a doable activity like arrive, go, lie, sneeze, sit, die, stay, remember, etc. Second, it will not have a direct object receiving the action. So typically the structure will be:
rester + preposition + phrase
la voiture est restée au garage
il rest dans le même quartier que nous
rester à lire jusque trad dans la nuit
Now back to the sentence les garçons restent des garçons. Here des is probably a contraction of de + les and the actual phrase is idiomatic
As PatrickJaye notes, "les ordres" could refer to orders in general or the specific orders we happen to be referring to.
However, the question remains, as "des ordres sont des ordres" is apparently the simple plural of "un ordre est un ordre", and need not imply "some" per se.
On that note, here's a discussion (where Sitesurf provides input) of "happy employees are good employees", which works with either les/des or des/des:
And here's a discussion of "boys will be boys", for which the only pattern currently accepted by Duo is les/des, even though it's arguably similar to "orders are orders" and it's hardly clear why les/les wouldn't work:
As you can see by all of that, there are different ways to think of these sorts sentences that might allow either article in either position, though nuances might make some combinations unlikely.
Apart from where such nuances might be specifically at play, it could be that some ways of saying these things just sound more natural to French speakers as a result of habit. Or it could be that not all of the possible options have been considered by the course contributors.
This combination, des/les, strikes me as unlikely, because the English doesn't really support this particular asymmetry. "Les/des" is not out of the question, and des/des also strikes me as a possibility as a simple pluralization of "un ordre est un ordre".