Oregon and Washington added value, just not enough. They ended up receiving half shares, and at that rate didn't actually cost any existing Big Ten member any money. I think they are due to get full shares at the next contract.
ESPN paid the ACC, a full share for SMU, Stanford, California so their addition wasn't dilutive. Their not getting a full share in return just made the deal economically better for existing members, particularly in the case of SMU.
Don't get me wrong. I'm not disagreeing with your point. I just think of the word dilutive as meaning that an addition would reduce value on a per school basis.