Ellicott comments as follows:
(19) A stranger—i.e., a foreigner in blood, who has been adopted into
the nation, received circumcision, and become a full proselyte. It is
not improbable that many of the “six hundred thousand” reckoned to
“Israel” (Exodus 12:37) were of this class—persons who had joined
themselves to the nation during the sojourn in Egypt, or even earlier.
(See Note on Genesis 17:13.) When the “exclusiveness” of the Hebrews
is made a charge against them, justice requires us to remember that
from the first it was open to those who were not of Hebrew blood to
share in the Hebrew privileges by accepting the covenant of
circumcision, and joining themselves to the nation. It was in this way
that the Kenites. and even the Gibeonites, became reckoned to Israel.
Similarly, The Pulpit Commentary says this:
Verse 19. - This is not a mere "vain repetition" of verse 15. It adds
an important extension of the punitive clause - "that soul shall be
cut off from Israel" - from Israelites proper to proselytes. We are
thus reminded, at the very time when Israel is about to become a
nation and to enter upon its inheritance of exclusive privileges, that
no exclusion of the Gentiles by reason of race or descent was ever
contemplated by God, either at the giving of the law, or at any other
time. In Abraham all the families of them were to be blessed (Genesis
12:3). It was always open to any Gentiles to join themselves to Israel
by becoming "proselytes of justice," adopting circumcision and the
general observance of the law, and joining the Israelite community.
The whole law is full of references to persons of this class (Exodus
20:10; Exodus 23:12; Leviticus 16:29; Leviticus 17:10; Leviticus
18:26; Leviticus 20:2; Leviticus 24:16; Numbers 35:15; Deuteronomy
5:14; Deuteronomy 16:11-14; Deuteronomy 24:17, 19; Deuteronomy 27:19;
Deuteronomy 29:11, etc.). It must have been largely recruited in the
times immediately following the exodus from the "mixed multitude"
which accompanied the Israelites out of Egypt (Exodus 12:38), and from
the Kenites who joined them in the wilderness (Numbers 10:29-31;
Judges 1:16). Born in the land - i.e., an Israelite by birth - "the
land" is, no doubt, Canaan, which is regarded as the true "Land of
Israel" from the time when it was assigned by God to the posterity of
Abraham (Genesis 15:18). Exodus 12:19.
Indeed, simple mathematics supports this idea. In the four generations (Gen 15:16) from Joseph to Moses, it would be impossible for the Israelite men to have expanded from 75 people to more than 600,000 without the help of foreign women and men, mostly from Egypt, such as the mixed multitude.