That is not my understanding. If a character is a custom lineage, he gets the custom lineage traits only. It would be way OP to give someone a +2 ASI, a feat, a free skill plus everything you get with Tiefling.My interpretation is you can "write in" any race with a custom lineage. If the player writes in "Tiefling" they can qualify for tiefling racial feats (but obviously not half orc racial feats).
You can be a custom lineage Tiefling, or a half orc that looks like a Tiefling or anything in between, but the only things you get are the things in custom lineage (+2 ASI, a feat or darkvision, a skill)