The WTS has discussed this more recently than 1989 (sorry I don't have the exact quote) but most JWs sleep right throught this point, new covenant (law covenant) = mediator Jesus (Moses) = spiritual Israelites (fleshly Israelites).
*** w03 2/15 p. 22 What Does the Lord’s Evening Meal Mean to You? ***
The parties to the new covenant are God and anointed ones. (Jeremiah 31:31-34; Hebrews 12:22-24) Jesus is the mediator. Made operative by Christ’s shed blood, the new covenant took out not only from the Jews but also from the nations a people for Jehovah’s name and made them part of Abraham’s "seed." (Galatians 3:26-29; Acts 15:14) This "everlasting covenant" provides for all spiritual Israelites to be resurrected to immortal life in heaven.—Hebrews 13:20.
*** w96 9/1 p. 14 The Law of the Christ ***
Who would be taken into this new covenant? Certainly not the literal "house of Israel," who rejected the Mediator of this covenant. (Hebrews 9:15) No, this new "Israel" would be the "Israel of God," a nation of spiritual Israelites. (Galatians 6:16; Romans 2:28, 29) This small, spirit-anointed group of Christians would later be joined by "a great crowd" from all nations who would also seek to worship Jehovah. (Revelation 7:9, 10; Zechariah 8:23) While not party to the new covenant, these too would be bound by law. (Compare Leviticus 24:22; Numbers 15:15.)
*** w92 3/1 p. 31 Questions From Readers ***
What about the new covenant between Jehovah and the nation of spiritual Israel? Jesus Christ had the glorious role of go-between, the Mediator between Jehovah and spiritual Israel.
*** w91 2/15 p. 18 "You Were Bought With a Price" ***
Especially since 1935 has there been a concerted effort to gather such ones. Revelation 7:9-17 describes them as "a great crowd" who owe salvation to God and to the Lamb. They still need to survive "the great tribulation" and be ‘guided to fountains of waters of life,’ for Revelation 20:5 shows that such ones will become fully alive, having perfect human life, by the end of the Thousand Year Reign of Christ. Those who then pass a final test in their perfect human state will be declared righteous for everlasting life on earth.—Revelation 20:7, 8.
Nevertheless, in a preliminary way, the great crowd have already "washed their robes and made them white in the blood of the Lamb." (Revelation 7:14) Christ does not act as Mediator of the new covenant toward them, yet they benefit from this covenant through the work of God’s Kingdom.
They get confused because 1 Timothy 2:5 says-
"There is only one mediator between God and mankind, himself a man, Christ Jesus."—1 Timothy 2:5.
And most JWs apply that to mediating prayer to God and since all JWs, anointed and great crowd, pray to God through Christ they assume Christ is the mediator for the GC too.
The WTS has applied it that way.
*** w75 7/1 p. 402 Coming to the Hearer of Prayer ***
To communicate readily with the heavenly Father, one must accept Christ Jesus as Lord. Peace with God can come only through the appointed way, the ransom provision by the Lord Jesus. (Matt. 20:28; 1 Tim. 2:5, 6) He is the provision for approach to God in prayer and reconciliation with him. Jesus rightly declared: "No one comes to the Father except through me." (John 14:6)
He is the "helper" that plays a vital role in our having access to the Hearer of prayer. (1 John 2:1, 2)
*** jt (2000) p. 13 What Do They Believe? ***
Prayers are to be directed John 14:6, 13, 14; 1 Tim. 2:5
only to Jehovah through Christ