I wasn't aware that they had changed 'her seed' to 'her offspring' so did a bit of research. It appears that they introduced this idea over 10 years ago in the Revelation climax book.
First, there is the seed, or offspring, of the Serpent. What is this? It surely includes those other spirit creatures who joined Satan in his rebellion and who were finally “hurled down with him” to the realm of the earth.
They then argue that 'her seed' refers to more than just one person (Jesus)
14 In the Bible, anointed Christians are called Jesus’ brothers, and as his brothers, they share the same Father and the same mother. (Hebrews 2:11) Their Father is Jehovah God. Hence, their mother must be “the woman,” God’s wifelike celestial organization. They become a secondary part of the seed, Christ Jesus being the primary part. The congregation of these spirit-begotten Christians on earth make up God’s visible organization that serves under his womanlike organization in the heavens, where they will be united with Christ Jesus at their resurrection. (Romans 8:14-17; Galatians 3:16, 29) Though not part of the seed, millions of other sheep out of all nations are being united to serve with God’s organization on earth
Why should that be a big deal for JWs? Because they have said this -
Furthermore, Jehovah foretold that a future “seed” would redeem mankind from sin. (1 John 3:8) W 2010 5/1
Did you tip the Brother well? The best tip would have been a glossy brochure advertising a real tour.