I keep seeing differing responses to that. Some have said that DME just has to be an ingredient.
I have two different plans based on which rule interpretation is correct. If some form of mashing is allowed, I'm planning to decoct a pound or two of grain, then get the remainder of my sugars from DME.
I'm not sure exactly what "challenge" you guys are talking about, but I can tell you, I have experience mashing extract as explained in the link that follows. A couple years ago, my local homebrew club ran an all ESB competition where we all brewed the same extract ESB recipe to see whose process reigned supreme or whatever. The ingredients list included aromatic malt, which fortunately contains some enzymes (supposedly, anyway). So, I did a quick steep of the aromatic and other specialty malts for a few minutes, removed the steeping bag, added all the extract (several pounds), then maintained the temperature at 150 F for like 40 minutes or so, then brought to the boil and brewed as normal. (Yeah, it was definitely single infusion.) The result? Well I can't say if it was due to the mash or due to my use of distilled water or anything else, but..... I won the gold medal out of about 8 or 9 entries. And I'm sure no one else did a glorified steep like I did. Otherwise all entries were exactly the same recipe, same ingredients, but just different water and different process and different brewers. It was pretty cool to see my efforts paid off.
So anyway.... read what follows if you've ever thought about mashing extract. It can be done, and it does seem like maybe it makes a difference.https://www.homebrewersassociation.org/forum/index.php?topic=14465.0
Apologies if I doth digress too much.