The Zohar brings another explanation to Isaiah 27:13. ‘those who were lost… will come and worship YHVH ’ are the righteous that is Yessod and the congregation of Israel that is Malchut. They are called ‘lost’ as we read in Isaiah 57:1
“הַצַּדִּיק אָבָד וְאֵין אִישׁ שָׂם עַל לֵב וְאַנְשֵׁי חֶסֶד נֶאֱסָפִים בְּאֵין מֵבִין כִּי מִפְּנֵי הָרָעָה נֶאֱסַף הַצַּדִּיק”
“The righteous man lost, and no one lays it to heart; devout men are taken away, while no one understands. For the righteous man is taken away from calamity;”
And about Malchut in Jeremiah 9:11
“עַל מָה אָבְדָה הָאָרֶץ נִצְּתָה כַמִּדְבָּר מִבְּלִי עֹבֵר”
“Why is the land (Malchut) ruined and laid waste like a wilderness, so that no one passes through?”
Assyria refers to the exile of the Israelites. ‘those who were lost in the land of Assyria’ is the congregation of Israel that comes back from exile.
“שִׁיר הַמַּעֲלוֹת בְּשׁוּב יְהוָה אֶת שִׁיבַת צִיּוֹן הָיִינוּ כְּחֹלְמִים.”
“A Song of Ascents. When YHVH brought back those who returned to Zion, we were like those who dream”
‘Return to Zion’ is the aspect of Yessod that is called ‘Zion’. YHVH returns Zion, Yessod, to its place to restore the connection of Yessod to the Congregation of Israel, Malchut.
Those ‘who were scattered in the land of Egypt will come and worship YHVH in the holy mountain at Jerusalem’ refers to those who were not with the Israelites that always have a connection to the Shechinah. After the return of Zion, they will be able to worship in the Holy Temple and connect with the Shechinah.