Why does it say Amar (singular)? The subject is Mo'av and Se'ir!
Radak: Mo'av was primary. This is like "va'Tdaber Miryam v'Aharon" (Bamidbar 12:1), "va'Tichtov Esther ha'Malkah u'Mordechai ha'Yehudi" (Esther 9:29). It uses feminine singular, for the female was primary. Se'ir is mentioned with Mo'av, for they were neighbors and rejoiced over Yisrael's Galus, like it says in Ovadyah.
Malbim: Mo'av did not take from Yisrael's inheritance. They only rejoiced over their fall; they joined with Se'ir.
Why do they say that Beis Yehudah is like all Goyim?
Rashi: Hashem's love for them ceased. He despised them to hand them over to Nebuchadnetzar, like other Nochrim.
Rashi citing Chazal 1 : They entered the Heichal and peeled off the forms of Keruvim that Shlomo drew on the Heichal wall "k'Ma'ar Ish v'Loyos" (Melachim I, 7:36 like a male embracing a female). They took them outside, and said '[Yisrael] show themselves to be modest - see what they engage in!'
Radak: Because Beis Yehudah remained after Yisrael were exiled, they thought that they will not be exiled due to the Beis ha'Mikdash amidst them. Now that they saw that they were exiled, they said 'Beis Yehudah was stricken like all Goyim.' All were stricken via Sancheriv and Nebuchadnetzar. Hashem's Shechinah among them did not help them!
Malbim: They are under nature; Hashem's Hashgachah departed from them. Chazal said (Eichah Rabah, Pesichah 9) that they took out the Keruvim, and said that Yisrael serve images, like all the Goyim - "Im Lo ha'Sechok Hayah Lecha Yisrael Im ba'Ganavim Nimtza" (Yirmeyah 48:27).
Part of this is in Eichah Rabah, Pesichah 9, but there it says like Malbim cites, that they showed that also Yisrael serve idolatry! (PF)
