If your event payload contains the whole updated state of the system, then yes, you need to know the ID of the original state in order to detect concurrent updates to the same original state. But if you want to keep things smooth for the user you'd have to merge those concurrently updated states, which may well be a massive pain, depending on your data model.
A better option is to only include the changes in the event payload, in which case you don't need to know the original state and you can simply project them all in order - and if one makes the next impossible, it's first come, first serve. Again, though, depending on your data model, generating an event payload that only includes the changed elements may also be a pain.
But you do not necessarily need to merge events. Contrived example, but say you have some kind of transactional flow (e.g. accounting) that is in the end signed/sent/processed by a central authority/job. There are two concurrent transactions modifying the same object. If the flow is to commit the latest state, then user1 can accidentaly commit user2's edits. This can easily happen if accountants connect with company credentials.
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For example there is an error in submitted (not yet committed) invoice for green and blue pens - invoice is for 5 boxes of blue and 5 boxes of green pens, but the courier departed with 11 total boxes, because it was a quick call directly to warehousing. Accounting does not particularly care for pen colour and get request to modify the invoice to total to 11 boxes. Accountant 1 modifies green pens count to 6, accountant 2 modifies blue pens. There is now high chance to sign the invoice with 12 boxes.
I understand that problem in example is solvable with checks and full state changesets, but it is a problem nevertheless. That's why I am proponent of history trees.
A better option is to only include the changes in the event payload, in which case you don't need to know the original state and you can simply project them all in order - and if one makes the next impossible, it's first come, first serve. Again, though, depending on your data model, generating an event payload that only includes the changed elements may also be a pain.