if so, doesnt that mean anyone can add to your collection?

Yes, anyone can 'add' to your collection, however, if you look at the metadata, https://1sat.market/outpoint/96f30dd3e27b683ecc10e1c9020b9385fb8f0e4b3c99e62a574558a50caa7b1e_0/inscription there is a sigma signature of the wallet that signed the ordinal. responsible marketplaces should check that all items claiming to be part of a collection have the same sigma signature as the collection origin ordinal. And creator/minter should sign the origin and all items with the same key. that way fakes can be dismissed/or not even presented.

perfect thank you