Checking claim in paper “A Read-Only Transaction Anomaly Under Snapshot Isolation” by Alan Fekete et al

Question

In the 2004 paper "A Read-Only Transaction Anomaly Under Snapshot Isolation" by Alan Fekete et al. (which can be found here and here) the authors state:

We also note that any execution of T1 and T2 (with arbitrary parameter values) without T3 present will always act serializably.

I have tested this claim inside PostgreSQL, like this

create table variables (
    name char(1) not null,
    value integer not null
);
insert into variables values ('x', 0), ('y', 0);

T2: begin
T2: set transaction isolation level serializable;
T2: select * from variables where name = 'x'
T2: select * from variables where name = 'y'
T1: begin
T1: set transaction isolation level serializable;
T1: select * from variables where name = 'y'
T1: update variables set value = 20 where name = 'y'
T1: commit
T2: update variables set value = -11 where name = 'x'
Connection reset. Reconnect (Y/n):

but as shown, when executing the T2's update, this transaction is aborted (I know this because the client, in the case pgcli, disconnected), which means that the schedule was not serializable.

What is happening here? Is the authors claim wrong? Is it a bug inside PostgreSQL? Or most probably, I made some mistake I am not aware of?

Thanks