Implied its in his posession. Implied he can be found sailing around on his ship. Not implied that he has territory or another place to put it. If Kidd wants to go after him to get to the RPG then its implied that the RPG is with him.
Nothing is certain and you should not take my post too seriously. Its clearly hyperbole.
The point is that what Shiryu said makes absolutely no sense when you realize that the WG is hiding several things very close by in MJ, but would hide the RPG far away from them.
The only real valid reason I can think of why they would keep the RPG away from MJ is that it can, for some reason, not be brought to MJ.
I understand your point & yes you're right, it's better that Poneglyph is hidden in Mary Geoise, not on some Pirate Ship, especially seeing all the things WG keep there, but i think it ties to his Ability,
Road Poneglyph could be indeed hidden in some Place or Room that belongs to WG & is guarded by them, but only way to access it is through this Man With Burn Scar hence why He is sailing around not letting anyone know his whereabouts
So even if you find Road Poneglyph's Location and take it from WG, you'd still need Man With Burn Scar & vice versa.
Basically WG guard the Chest/room containing Road Poneglyph while Man With Burn Scar guards the Key, and keeping them Separate is Best Plan
I think his Whirlpool Ability is reference to Davy Jones Locker.
I also wanna point out that Giant Straw Hat isn't necessarily in Mary Geoise,
Cuz after Gorosei were revealed to be able to Teleport & their Powers seem to be taken from Imu himself, then it's possible Imu can summon himself to that Mysterious Locker, meaning it could be anywhere in the World, not necessarily Mary Geoise