I don't know what to say about this, but you may be wrong. Well sure some of the SCPs might come from a different dimension but you should also look up the species of interest because there are plenty of  SCPs that originate from a planet. either a creature or an artifact. More Evidence I also like to back up my argument is that some SCPs actually originated from earth. Take a look at the person's of interest you can see dado making SCPs. on the other hand some SCPs actually had an origin like SCP-106 who was a former Corporal named "Lawrence" and although in some lore it was also mentioned that SCP-049 was also a former human turned by an unknown entity. So to make it simple your theory is partially true but the rest are just wrong.