This is true, but not that much.
Officially we recommend using multiple documents, but some people like to keep everything in one document either because Search Everywhere results are not currently editable or out of habit (just my guess). We respect that too.
If you want to share part of your huge document though, you'll need to make that a separate document in Dynalist.
The problem with archiving in an outliner is losing context. For example, maybe you have an item that says "Hand in final draft" under "Essay for some course". If you archived "Hand in final draft", it has lost its context. We don't know much value is preserved this way.
Personally, I just use the "Delete checked items" to clear my items from time to time as I don't need archiving. If you're fine with losing the context, we can add a similar option that sends all checked items to a location. The UI will be similar to the "Move to..." dialog.
What do you think?