If it still smells that bad then yeah there's likely to still be some left.
You could perhaps use a squeegee on a long-handle to drag some out but suspect as you say it's down the hole.
Is it onto concrete then?
To clean it off i'd be using a soft brush with a longish handle, and putting a bleach solution on every surface under there, and probably capping it off as you suggest when it's dried & you're sure you've got as much out as you can.
It's a bit odd it had a dump there, are the cats getting on ok together? Maybe one of them's not getting on well outside so it didn't want to go there?
One of our cats always used to go anywhere except the litter tray, only stopped when we let her go outside.