Another interpretation: Pharaoh spoke, yet did not know what he was saying. He saw that Israel were destined to weep in the wilderness, as it is said (Numbers 14:1), "And all the congregation lifted up" [their voice and wept]. [His boast against Israel was in truth a prophecy he did not understand, pointing toward the night of weeping after the report of the spies.]
Pharaoh Unwittingly Foretells Israel Weeping in the Wilderness
Yalkut Shimoni on Torah 230:8
דָּבָר אַחֵר, אָמַר פַּרְעֹה וְלֹא יָדַע מָה אָמַר, רָאָה שֶׁעֲתִידִין יִשְׂרָאֵל לִבְכּוֹת בַּמִּדְבָּר, שֶׁנֶּאֱמַר (במדבר יד, א) "וַתִּשָׂא כָּל הָעֵדָה".