I can't say I understand the example. It seems to talk a lot about implementation details compared to the naive SQL:

    create table account (
        account_id bigint primary key,
        balance bigint not null default 0,
        check (balance >= 0)
    );
    
    begin;
    update account set balance = balance - $amount where account_id = $from_id;
    update account set balance = balance + $amount where account_id = $to_id;
    commit;

Who hurt you?