nope, now you can only install the stored modules in the stored modules section, there are no combined view with all available for a slot. in outfitting you can buy a module and install it, but if you want to install a stored module in a module slot you need to exit outfitting enter stored modules, select the module you want and finally select the slot.
In horizons works exaclty like you say, you select a slot and can install any stored module or buy new module in the same place.
The almighty Rainbow Snakes finally granted me access so I could try for myself and you're right. During the alpha I found and documented a workaround for a bug that prevented this from working properly, but that no longer works and it appears as if that functionality was removed altogether. And due to the alpha forum getting "archived" into oblivion I can't even link to that post to illustrate what I meant...

Basically one would go to "Ship Loadout", select a slot, click "shop for module", and then have stored modules mixed in with whatever is available for purchase. Apart from the bugs it was a clear improvement over the old UI with less clicks and less searching/scrolling. All I got left is a screenshot I made:
That was the combined view working as expected, the highlighted Experimental category contained only a Flak Launcher I had in storage at that station, after clicking through I was then able to install that into a hardpoint without ever having to leave the Ship Loadout section.
Obviously there is still a lot of work left for FD (as is tradition for a Elite DLC launch

) but atm this looks to be worse than what we had in alpha...
Well, at least I found where they've hidden the engineering info: