Why the Egyptians were engaged in Aveilus and not Aninus

Just something I saw on Shabbos, in Rabbi Matis Blum's Torah Lodaas parsha sheet. Towards the very beginning of Masei:

ג וַיִּסְעוּ מֵרַעְמְסֵס בַּחֹדֶשׁ הָרִאשׁוֹן, בַּחֲמִשָּׁה עָשָׂר יוֹם לַחֹדֶשׁ הָרִאשׁוֹן: מִמָּחֳרַת הַפֶּסַח, יָצְאוּ בְנֵי-יִשְׂרָאֵל בְּיָד רָמָה--לְעֵינֵי, כָּל-מִצְרָיִם.	3 And they journeyed from Rameses in the first month, on the fifteenth day of the first month; on the morrow after the passover the children of Israel went out with a high hand in the sight of all the Egyptians,
ד וּמִצְרַיִם מְקַבְּרִים, אֵת אֲשֶׁר הִכָּה ה' בָּהֶם--כָּל-בְּכוֹר; וּבֵאלֹהֵיהֶם, עָשָׂה ה' שְׁפָטִים.	4 while the Egyptians were burying them that the LORD had smitten among them, even all their first-born; upon their gods also the LORD executed judgments.

Rashi writes:

4. And the Egyptians were busy burying because the Lord had struck down their firstborn and had wrought vengeance against their deities.		ד. וּמִצְרַיִם מְקַבְּרִים אֵת אֲשֶׁר הִכָּה יְ־הֹוָ־ה בָּהֶם כָּל בְּכוֹר וּבֵאלֹהֵיהֶם עָשָׂה יְ־הֹוָ־ה שְׁפָטִים:
And the Egyptians were busy burying: occupied with their mourning.		ומצרים מקברים: טרודים באבלם:

Now as for the dvar Torah:
I am almost certain he meant it as a joke, and at the very least I hope so. But it is quite amusing. Check it out, to the right.