First, doing it manually through the app is not okay since it does not scale, you don't want to read a message, you want to retrieve and index all messages, and you might want to process many devices quickly.
Second, apps usually do not show the user all the information that's available - often there is extra metadata (which may be as important as the message contents) so you do want to decode the actual message database.
Third, doing it through the app might change things - the app may change state (for example, mark an unread message as read), send some notification to central servers, alter metadata, etc. So it potentially disrupts evidence, and that's not okay.
So the original blog post from Cellebrite makes all sense - if you do want to do forensics, then a tool that does all that is really a requirement, it's not wasting time.