Believers in false churches
Isaiah 55:10-11 is often used to justify the claim that "there must be at least some saved souls even in false churches, as long as the Gospel is taught there." But God uses His Word for many purposes, including fulfillment of promise (Matthew 24:14), teaching, rebuking, correcting, and training in righteousness (2 Timothy 3:16). Given that we don't know God's specific purpose for His Word in any given church at any given time, isn't it more correct to say that we simply don't know if there are any saved souls in any given case like this, and leave that knowledge to God?
I do not know to whom you attribute the quotation that is connected to Isaiah 55:10-11. Rather than proving the existence of believers even in false churches, Isaiah 55:10-11 illustrates in figurative language that God’s word is not spread without effect; there will be always be results when God’s word is proclaimed. Either people will be brought to faith in Christ or strengthened in the faith, or people will be hardened in their unbelief.
A Bible verse like Romans 10:17 (“Consequently, faith comes from hearing the message, and the message is heard through the word about Christ. “) tells us that we can expect to find God’s kingdom wherever the gospel of Jesus Christ is proclaimed in truth and purity, and the sacraments are administered according to the gospel of Jesus Christ.
Finally, as you indicated, God alone knows who belongs to his kingdom because God alone can see what is in the heart (1 Samuel 16:7; Luke 17:20-21; 2 Timothy 2:19).