network of lightning – Eltoo eliminates the need to monitor the blockchain / implement watchtowers

In the introductory article Blockstream on Eltoo, a paragraph says:

[..] virtually all participants share a common set of transactions, unlike LN-penalty, which requires an asymmetry in which the participant has access to which transactions, in order to tailor the reaction to the misconduct. This change eliminates what we call toxic information in Lightning. The toxic information comes from operations owned by obsolete states, which, in the event of a leak, would result in a loss of funds. This happens not only if a part behaves badly, but also if a node forgets an update (for example, when restoring from a backup). With eltoo, this is no longer possible because only the agreed states can be set (that is, eloo is without penalty).

This seems to imply that it is not possible to broadcast an old string state and have it accepted by the network, which would greatly simplify the design issues we face today. I strongly feel that I do not understand something well and that I would very much like to get clarification from someone who is more aware. Thank you