well there are no european cores outside europe, except british india.
eg when you look at the french or dutch inc files, you'll find that everything out of europe is disabled as national province. same for the british empire, except india. same for US islands in the pacific, same for japanese territories in northern china.
a core is defined as a national province, according to the inc files. the reason i oppose that is not necessarily the claim on it, but the fact that - once seceeded - one gets 100% of everything.
of course if one argues ahistorical (and theres an event for polish liberia in core, too
), you could state that large parts of Brasil is german because of Humboldts journeys
.
seriously, its not a game breaker but adding a core is more than only having claims on it, gameplay wise. its a huge difference if you simply controll and own a province (like french algeria) or - on top of that - have the national rights for these provinces (which makes you much stronger than before).
so theres no real room for interpretation, the command is wrong. secede_province works fine and is fully correct, but add_core is biased (unless you add cores for all the owned + controlled territories, ie JAP, USA, FRA, ENG, ITA, HOL, BEL, POR, SPR/SPA, AST).
if one follows your definition, that would be the logical consequence as these territories were in possession for long in some cases.