DEVONtechnologies didn’t create the text editing code for rich and plain text documents. Apple did.
I’m puzzled by your comment that the Tab key doesn’t insert a tab when editing a plain or rich text document. It does here, in DEVONthink and in DEVONnote. Editing text documents in DEVONthink behaves essentially like editing such documents in TextEdit using wrap to window. In the case of rich text documents hyperlinks, images, lists and tables can be added.
However, the Tab key in conjunction with other keys has other functions when not in edit mode, such as to shift focus, e.g., among elements of the Three Panes view. That’s not bad or inconsistent UI.
DEVONthink has been available since 2002, and the developers have often responded to user requests for features. In a previous version a warning was given when Delete was invoked on an item. Power users objected to the “nanny” warning. DEVONthink then provided a failsafe for deletion without warning (including UnDo if selected immediately). Instead of being sent to the System Trash, deleted items are sent to a database Trash and remain there until the user invokes a command to empty the database Trashes. Note that items in a database Trash remain searchable until flushed to the System Trash and can be refiled if desired.
In the case of Indexed items, when the database Trash is emptied the user will be given a choice to remove only the database records, or to also remove the originals of the items in the Finder.
Every couple of years users have again raised the issue of warnings when items are deleted, protesting them if they are present, or asking for them if they are not displayed.
As it happens, the next maintenance release of DEVONthink will display a confirmation request when Delete is invoked. For power users who don’t like that, a checkbox is provided to prevent future display of the message.
A comment about the UI problems of warning or confirmation messages. They often turn out not to protect well against inadvertent, accidental, or “What was I thinking?” actions, because users become accustomed to automatically clicking on the button to confirm the action. That’s why DEVONthink added database Trashes, where the documents remain searchable, viewable and recoverable, as more protective in the long run.