If Jesus did not specify heavenly angels, his meaning would have been far more ambiguous. The word "angel" means "messenger." It does not always refer to heavenly beings, but can refer to humans as well.
And Jacob sent messengers before him to Esau his brother unto the land
of Seir, the country of Edom. (Genesis 32:2, KJV)
וַיִּשְׁלַ֨ח יַעֲקֹ֤ב מַלְאָכִים֙ לְפָנָ֔יו אֶל־עֵשָׂ֖ו אָחִ֑יו
אַ֥רְצָה שֵׂעִ֖יר שְׂדֵ֥ה אֱדֹֽום׃ (Genesis 32:3, TR)
The word "מַלְאָכִים֙" is the Hebrew word for angels, but is translated here as "messengers." Jacob could not have sent heavenly angels to his brother, right?
And I sent messengers out of the wilderness of Kedemoth unto Sihon
king of Heshbon with words of peace, saying, (Deuteronomy 2:26, KJV)
וָאֶשְׁלַ֤ח מַלְאָכִים֙ מִמִּדְבַּ֣ר קְדֵמֹ֔ות אֶל־סִיחֹ֖ון
מֶ֣לֶךְ חֶשְׁבֹּ֑ון דִּבְרֵ֥י שָׁלֹ֖ום לֵאמֹֽר׃ (Deuteronomy 2:26, TR)
Again, this is the word "angels" in Hebrew, which has been rendered as "messengers" in English. The original language did not distinguish between the two.
This usage of "angels" to reference "messengers" also occurs in Judges 6:35, 2 Samuel 3:26, 2 Kings 17:4, Nehemiah 6:3, and Jeremiah 27:3.
It is more often found in its singular form, however.
מֲלְאָךְ mălʼâk, mal-awk'; from an unused root meaning to despatch as
a deputy; a messenger; specifically, of God, i.e. an angel (also a
prophet, priest or teacher):—ambassador, angel, king, messenger.
Translation counts in the AV (Authorized Version, i.e. KJV):
- AV — angel 111, messenger 98, ambassadors 4, variant 1
The Greek word is the same story. For example:
And when the messengers of John were departed, he began to speak
unto the people concerning John, What went ye out into the wilderness
for to see? A reed shaken with the wind? (Luke 7:24, KJV)
ἀπελθόντων δὲ τῶν ἀγγέλων ἰωάννου ἤρξατο λέγειν πρὸς τοὺς ὄχλους
περὶ ἰωάννου τί ἐξεληλύθατε εἰς τὴν ἔρημον θεάσασθαι κάλαμον ὑπὸ
ἀνέμου σαλευόμενον (Luke 7:24, TR)
ἄγγελος ággelos, ang'-el-os; from ἀγγέλλω aggéllō (probably derived
from G71; compare G34) (to bring tidings); a messenger; especially an
"angel"; by implication, a pastor:—angel, messenger.
- Translation counts for ἄγγελος: AV — angel 179, messenger 7
This shows that the word "angel" can mean simply "messenger" and be applied to the human variety quite regularly.
Jesus' specification of the angels as being "of heaven" or "in heaven" is a necessary clarification, because the word's meaning would not otherwise be limited to extraterrestrial (non-human) beings.
Both in Hebrew and in Greek, the word "angel" can refer to a human: it simply means a messenger. Jesus used the modifier "in heaven" to specify to which variety of messenger he was referring. The angels "not in heaven" might have been those of the human variety--which is why Jesus needed to be more specific.