It doesn't ever say it isn't, and everything it does ay on the topic is that it was through lineage. Paul speaks of gentiles being grafted to Israel, but never states the grafting is to specific tribes.
Everything in the law might, but not everything written does because the prophets wrote that converts would be considered part of the tribe among which they dwelt. Tribal membership initially was determined by natural heredity, but the prophets make clear that one day (it shall come to pass) it would go beyond that into including gentile converts.
Only those of the tribes of Israel could receive an inheritance in the land, because the land was alloted according to tribe; and Ezekiel 47:22-23 makes clear that converts would receive an inheritance in the land, so simple logic proves they will be members of the tribes among whom they dwell.
This passage makes clear that the land inheritance via tribal membership would be to both converts and their children. This is obviously referring to the new covenant when all natural barriers and distinctions would be removed and all in Christ are treated equally. IMO tribal membership in the new covenant has nothing to do with land inheritance and everything to do with spiritual inheritance. But the prophets didn't understand such concepts and related everything they saw in visions and the spirit to the land of Israel. Thus the language
And it shall come to pass, that ye shall divide it by lot for an inheritance unto you, and to the strangers that sojourn among you, which shall beget children among you: and they shall be unto you as born in the country among the children of Israel; they shall have inheritance with you among the tribes of Israel. And it shall come to pass, that in what tribe the stranger sojourneth, there shall ye give him his inheritance, saith the Lord GOD. Ezekiel 47:22-23