All it comes down to is what kind of body is being described in each instance. That is all.
The Ephesians..'there is one body...' is about the Ephesians church. As an example of unity in a church to other churches as the letter was distributed around.
It's not saying there is 'only one' body.
Again..1 co 12 is describing a local body. All redeemed do not meet..are not connected.. one member does not feel the hurt of another when one is in Africa and another in Australia.
An arm..is akin to an individual member of the body...not an entire supposed denominational wing like 'the church of America '
I could go on...
As for Landmarkism.. any church that holds to the distinctives of NT churches is one of God's churches. They may not have Baptist in the name .
But it has been mainly Baptist churches that have long tenaciously held to these distinctives.
My Church by J M Moody shows this better..and The Battle for Baptist history by I.K.Cross among others
The text does not say we were all baptized by one Spirit into water. It says “For by one Spirit are we all baptized into one body,” (1 Corinthians 12:13). The word “Spirit” here signifies the type of baptism that is being spoken of here. The Spirit baptizes here and not men. So this is a spiritual baptism. To read water baptism into this verse is to distort what the verse actually says.