Harvard knows that a DE class is much faster-paced and requires more independence than a regular HS class, but it also knows that its classes are way more advanced. Therefore, it is impressed with DE classes for admission purpose, sometimes for placement purpose, but not necessarily for credit purpose.
(Note that the situation is similar for AP classes. Many do not bring credit, some bring placement.)
AP as “default” level: the majority of students admitted to Harvard have had DE or AP classes. Those classes aren’t supposed to replicate 1st year classes at Harvard, but represent the basic level BEFORE Harvard.
Students are not penalized for attending a low-performing HS, but are supposed to attend a summer program to help them integrate into the academic demands of the college (as well as socialize and get comfortable using all the resources at their disposal.)
This is getting way off topic, but it actually is possible. It is exceedingly rare, but possible. As in [Evan O’Dorney](Evan O'Dorney - Wikipedia) rare. Still, @NavalTradition is correct; students entering Harvard with MVC/LA experience often take Math 23/25/55 in lieu of Math 21.
Getting slightly back to topic, there is no evidence to suggest OP, who remember is a freshman, is targeting HYPMS-level schools.
I am not interested in HYPSM schools. The only Ivy I might consider is Cornell, but that’s still a high, high reach. I have no idea what I want to major in, so that will affect what I apply to, but getting into a highly selective school isn’t that important to me.