What I got when he said that was not that he knew who or what the WG is, but now he knows what Robin is afraid of. That her enemy is not Spandam, or CP9, or even the WG as a whole, but more the fear of the WG's power. I thought the burning of the flag was a global "come on" to WG and showing Robin that he is not afraid of anything they can throw at him, and Robin shouldn't be either, thus saying he will never get tired of Robin or betray her.
The difference is slight, but is a clear difference. Mugiwara_no_Ice's post had the same idea.
You need to put the whole thing in its context
he was answering spandam
he just says that he understands the force and nature of the WG and that
he s not afraid of it–> he proves it by burning the flag!
It's not the WG itself, but more the force it represents that is frightening Robin, 'force of nature' as he put it.