There’s no reason to just “use Finder and forget about DEVONthink”, and DEVONsphere is not necessary if all you want is to have database items searchable in Spotlight/Finder. Mine show up fine.
This is a setting on a database level set in the Database Properties... window. At the very top there is an on/off checkmark for Create Spotlight Index
You find this either by right-clicking a database or in the menu at File > Database Properties. (The sidebar needs to be visible)
Where did you create those notes? If they don’t show up in a Spotlight search, I’m assuming it was in the Global Inbox. As far as I’m aware the default is off for the Global Inbox, but on for all other databases. There might be reasons why you would want to include or exclude database items from the Spotlight Index, so the option is nice.