Why does he need an Observatory for defense?
If you look at the symbols where an Observatory has its info without clicking the ? mark on mobile or hovering over on Desktop it looks exactly the same as a city defence symbol
So obviously this guy doesn't know how a observatory works and needs to learn from someone that does (or hover/click on ? to see the description)
For that matter attacking GBs also re-use the same symbols for attacking armies with the amount of swords being the only real defining difference until the Terra came out which uses the defensive layout of the symbol and adds blue
It's all different enough for attack and defence that if you know what you're looking at you know what it does. But the observatory doesn't have any difference between that and a event buildings defence army bonus
If I didn't know the pond was defensive army and only knew how the Observatory bonus works I could potentially mistake it as support pool. Likewise if I only knew the pond and didn't hover or see the ? symbols info on mobile I could easily mistake the Observatory as defensive pool bonus.
Now being desktop mainly player myself I've never had that problem as I'd be hovering just to access my Observatory. But this guy doesn't have an observatory yet so depending on what the mobile screen shows when looking at other players GBs I can understand the mistake
And this is all making a pretty big assumption - that if they ever saw the description for support pool they understood what they read and didn't just assume due to the symbol that it also does something for your own defence army