"That first covenant was between God and Israel and that first covenant did not include the Gentile nations.", however based on this Isaiah passage, it was inviting the Gentiles who were living in the land of Israel to join in the Shabbat (aka Sabbath). In fact, the entire force of the passage depends on the fact that these foreigners are not circumcised, not proselytes, and not becoming Jews.
Part of what you said above is accurate as shown below.
That first covenant was between God and Israel and that first covenant did not include the Gentile nations.
Then you misunderstood what Isaiah 56 is saying.
If you read the text below from Isaiah 56 this should clarify what you misunderstood.
Isaiah 56:6
Also
the foreigners who join themselves to the Lord, to minister to Him, and to love the name of the Lord.
To be His servants, every one who keeps from profaning the sabbath
and holds fast My covenant.
Isaiah 56:7
Even those I will bring to My holy mountain and make them joyful in My house of prayer.
Their burnt offerings and their sacrifices will be acceptable on My altar.
The codified law strictly identifies the condition that any Gentile sojourning in Israel must perform. In order
to be a member of that covenant between God and Israel, Exodus 24.
That condition requires the male to be circumcised.
Then that Gentile
is like a Jew but is not a Jew and
is in covenant to obey the codified law of Exodus 24.
Exodus 12:48-49
But if
a stranger sojourns with you, and celebrates the Passover to the Lord,
let all his males be circumcised,
and then let him come near to celebrate it; and
he shall be like a native of the land. But no uncircumcised person
may eat of it. The same law shall apply to the native as to the stranger who sojourns among you.
A Gentile living outside of Israel is not permitted to join the covenant of Exodus 24.
A Gentile who is uncircumcised and living outside of Israel cannot follow the codified law stated in Exodus 24.
because they are not members of the covenant, they are not legally bound to the codified law.
If you really want to perform the offerings, the sacrifices, the Passover, the Sabbath. Then you you must join
that covenant in Exodus 24.
That's what the text states.