In that game term and services it basically says if you make a mod for this game and you release that mod on this platform/or make it public in general, it's basically become free for all other modder to edit and modify it, that's why there is so many unofficial versions, forked version, improve version, etc. And the game dev can add mod features without modder permission.
Where did you read that? Because the Workshop EULA specifically says that only Valve or the developer can make a fork of a mod, and only to make it compatible with changes to the workshop for Valve, or to "enhance the gameplay experience" for the dev.
EULA section:
B. Content Uploaded to the Steam Workshop
Some games or applications available on Steam ("Workshop-Enabled Apps") allow you to create User Generated Content based on or using the Workshop-Enabled App, and to submit that User Generated Content (a %u201CWorkshop Contribution%u201D) to one or more Steam Workshop web pages. Workshop Contributions can be viewed by the Steam community, and for some categories of Workshop Contributions users may be able to interact with, download or purchase the Workshop Contribution. In some cases, Workshop Contributions may be considered for incorporation by Valve or a third-party developer into a game or into a Subscription Marketplace.
You understand and agree that Valve is not obligated to use, distribute, or continue to distribute copies of any Workshop Contribution and reserves the right, but not the obligation, to restrict or remove Workshop Contributions for any reason.
Specific Workshop-Enabled Apps or Workshop web pages may contain special terms (%u201CApp-Specific Terms%u201D) that supplement or change the terms set out in this Section. In particular, where Workshop Contributions are distributed for a fee, App-Specific Terms will address how revenue may be shared. Unless otherwise specified in App-Specific Terms (if any), the following general rules apply to Workshop Contributions.
* Workshop Contributions are Subscriptions, and therefore you agree that any Subscriber receiving distribution of your Workshop Contribution will have the same rights to use your Workshop Contribution (and will be subject to the same restrictions) as are set out in this Agreement for any other Subscriptions.
* Notwithstanding the license described in Section 6.A., Valve will only have the right to modify or create derivative works from your Workshop Contribution in the following cases: (a) Valve may make modifications necessary to make your Contribution compatible with Steam and the Workshop functionality or user interface, and (b) Valve or the applicable developer may make modifications to Workshop Contributions that are accepted for in-Application distribution as it deems necessary or desirable to enhance gameplay.
* You may, in your sole discretion, choose to remove a Workshop Contribution from the applicable Workshop pages. If you do so, Valve will no longer have the right to use, distribute, transmit, communicate, publicly display or publicly perform the Workshop Contribution, except that (a) Valve may continue to exercise these rights for any Workshop Contribution that is accepted for distribution in-game or distributed in a manner that allows it to be used in-game, and (b) your removal will not affect the rights of any Subscriber who has already obtained access to a copy of the Workshop Contribution.
Except where otherwise provided in App-Specific Terms, you agree that Valve%u2019s consideration of your Workshop Contribution is your full compensation, and you are not entitled to any other rights or compensation in connection with the rights granted to Valve and to other Subscribers.
(also bold of you to assume this game will get Workshop support given how it would basically break your save every other week because you can't block mod updates. Oh and the requirements on the game code are apparently not insignificant either)