That's another way of saying that only the modifications and derived works that Steemit Inc. permits are allowed. That's essentially the definition of proprietary software. The copyright owner can always give permission. It is the lack of necessity of permission to modify and use independently that defines open source.
Or to be more clear perhaps, it doesn't meet most generally-accepted definitions of open source. For example, it violates clause 6 of the Open Source Definition by discriminating against usage (without written permission) in the field of cryptocurrencies and blockchains.
You have your business reasons for the license, which is entirely legitimate, but as a trade-off for maintaining that position you are alienating those who prefer to be involved with true, permissionless, open source projects.