The verse you've quoted is about conversion. A change in status from being a stranger obligated by the land's law covenant to being a convert, a sort of adopted israelite. The fact remains the Old Covenant was with Israel. This really has nothing to do with who is in the new covenant. As I said earlier, God knows his own. It doesn't matter what the watchtower says, believes or teaches. Noone who God saves loses his salvation because of a human teaching.
For the sake of accuracy, even the watchtower teaches that God saves those not in the new covenant. They use 1 john to teach that. On that point i have no argument. God saves through his son. It's that concept that matters to me.
I don't think we can use the old covenant to mirror the new. The new covenant is specifically 'not like' the old one. The old convicted of sin. The new brings salvation. Watchtower teaching is that it is the basis for the salvation of all. Fine. I agree with that. Do I think the parties to the new covenant were limited to a spiritual israel and God with Jesus as mediator? I don't know. The answer rests in what the israel of god is. I'm not sure i see that point as essential anyway. What is essential is God's will to save. No doctrine of men can change that or determin who will be saved or to what destiny.
The two classes of saved argument is an old one, extending back to the 19th Century. I've read the arguments. I know Watchtower doctrine. I remain skeptical of it all though I have my personal preferences. Personal preferences, personal understandings, should remain personal unless they are backed by scripture. I'm unsettled, so I'll leave them unexpressed. However, one should note that benefiting from the old covenant did not make one a party to it. Captive slaves, while not parties to the Law Covenant, benefited in many ways. They were freed from religious prostitution. Their children wouldn't be burned in Molech's furnace. But they were not parties to the covenant.
Your slavation does not depend on your (or anyone else's) understanding of a covenant. It depends on God who saves us through Christ.