How did you arrive that that conclusion?
History! Most of the men there had multiple elders. There were a few who did not, but from my research, they did. My book is in another state, but I believe Sam Waldron's commentary on the 1689 discusses this concerning chapter 26 of the Confession. I know I have heard Dr. Waldron lecture on this and he would attest that most had multiple elders. Nehemiah Cox's church had multiple Elders. Nehemiah was one of the editors of the LBC 1689. Thus, part of the intrinsic foundation of this document was built by people who adhered to plurality.
John Bunyan's church, where he was Pastor, had multiple Elders. The oldest surviving Baptist church building in England, as well, is noted for having a multiplicity of Elders. Benjamin Keach, one of the most famous 1600's Baptist, supported multiplicity of elders, but did distinguish that from plurality. I think his disagreements with the word is extremely minor in today's debate, he believed they were equal but, like me, would have said that it is not necessary for a church to be a legitimate church. I think plurality would not have been an issue for him today as most agree with him.
The first President of the Southern Baptist Convention said, "In review of these scriptures we have these points clearly made. One, that over each church in the New Testament era a plurality of elders was ordained who were designated by the terms elders, bishop, overseer, pastor with the authority of the government of the flock of God.” He later observed of his own era that in general, "“A plurality in the bishopric is of great importance for mutual counsel and aid, that the government and edification of the flock may be promoted in the best manner.”
The Philadelphia Baptist Association did affirm a more defined ruling and teaching Elders while the Charleston Baptist Association (1774) defined the church with a localy "presbytery" but did not differentiate between teaching and ruiling elders, but embraced more of a multiplicity.
William Williams, one of the founding faculty of the Southern Baptist Theological Seminary wrote in 1874 that “In most, if not all the apostolic churches, there was a plurality of elders."
On and on... plurality was used by historic and respected Baptists throughout history.