You would need to use serializable isolation for this to hold true. Any isolation level less than serializable will use the snapshot that was active at the time of the select.
In Postgres, even with the serializable isolation level, all transactions that touch the same rows must also be using the serializable isolation level or it's not really enforced. This is one aspect of serializable isolation in Postgres that seemed like a major gotcha for real world application development. There's no future proof solution: new code can be added that doesn't use the serializable isolation, and then the assumptions of isolation from the earlier code are no longer valid.
In Postgres, even with the serializable isolation level, all transactions that touch the same rows must also be using the serializable isolation level or it's not really enforced. This is one aspect of serializable isolation in Postgres that seemed like a major gotcha for real world application development. There's no future proof solution: new code can be added that doesn't use the serializable isolation, and then the assumptions of isolation from the earlier code are no longer valid.
FOR UPDATE is the only real solution in my eyes