Here goes;
EU is a club with 28 members. The object of the club is to allow the members to swap among themselves more easily than before.
Anyone can apply to be a member, but all other members must agree. Bit like enrollment in the Free Masons. However, the leader of the Club is not elected by the individuals at the bottom of the pile of each of the members.
10 pay money into the club and the other 18 take money out of the club in addition to the swaps that they make.
Of the 10 members that pay money in to the club one member wants to leave the club after the people of that club voted to leave the club. The club rules do not require members to explain why they want to leave.
The 18 members who take money from the club do not want any of the 10 members who pay into the club to leave as their free money may be reduced.
The other 9 members who pay money into the club do not want the other member who pays in to leave as they may have to pay in more to the club so that the 18 who take money out of the club receive the same as before and do not get bored and want to leave the club as their free money has reduced.
Two of the club members made a deal by mutual consent in 1998, but did not involve the other members of the club. So the deal is not governed by the club rules.
If a club member wants to leave the club it must follow the club rules that were made in 2009. The leave rules are leave with an agreement if possible. If not leave without a agreement and the previous club rules do not apply to the member who voted to leave.
Whether or not the decision by one member of the club to leave the club has advantages or disadvantages for any of the other members of the club does not prevent the member who voted to leave from leaving the club. All members of the club agreed this principle when they all signed the clubs rules for leaving the club in 2009.
Deals made between club members individually do not change the club rules of leaving the club.
Because 9 members of the club who pay money into the club are upset that the other member who pays into the club wants to leave in accordance with the club rules agreed by all 28 members of the club. So they are trying to cheat by saying that the member who wants to leave must follow rules made in a deal that did not involve the club at all and for which the leaders of the club were not a signatory.
The member of the club who wants to leave was once leader of a large club that spanned the World and had more members than the EU does today, but did not object when other members of the club chose to leave. That member understandably expects the same response from the members of the new club they joined in 1973 to behave in the same manner.