Rabbi Shlomo Kluger in his sefer Imrei Shefer here gives a very different explanation of this posuk which includes explaining the last phrase literally - that the Yisrael will leave the land.
The full English translation of this piece can be found here, but I will quote a section of it:
It seems that all that Pharaoh wanted was to get rid of
Yisrael’s presence from his land. However, he was concerned about
exiling them from his land, because that would require fighting with
them, provoking them to resist and fight back, and since they were more
numerous than the Egyptians, they would not succeed against them.
Therefore, he calculated that since he was the king he
big was his population and that of Yisrael, and he knew that they were
more numerous than his people. But they, on the other hand, did not
know how big the Egyptians were, and on the contrary, they probably
thought that the
Egyptians were more numerous than them.
Therefore, Pharaoh counselled that they should start
to impose decrees against Yisrael to make it seem as if they are
and afraid about the future - afraid that Yisrael’s population might
increase. Then Yisrael will think that if they are afraid of an
increase only in the future, that implies that right now they are not
afraid. If so, it must be that they know that we are fewer in number
than them. This 'knowledge' will persuade them to accept our decrees
and not stand up against us, and since the decrees will be harsh they
will flee the country of their own accord.
However, in reality they cannot flee from Egypt, because it is
impossible for a slave to escape, because, as Chazal have taught, the
used sorcery so that no slave could escape from Egypt. But this is only
true as long as they were in Egypt. But if a slave went outside of the
country, from there he could flee. Therefore, Pharaoh said, if we place
upon them harsh decrees which would make them want to leave, then
when a war will befall us they will act as if they were our friends,
and go together with us out of the country to fight against our enemy,
and once they have left the country they
will flee of their own accord, and that is what we want. According to
this, the words “they will fight בנו” means
“they will fight amongst us”.
This is what the Torah is saying: “Behold the people of the children of Israel are numerous, and stronger than us”, and I am afraid to oppose them, to fight against them in order to exile them. Therefore, “Be prepared, let us deal shrewdly with them”. And how will we deal shrewdly? - “lest they become numerous”, that is, we will act as if we are worried that they might become numerous in the future, but that right now, we are not afraid, because we are more numerous. Thus they will accept our decrees, and not oppose us. Thus, “it will be when a war befalls us, they too will be added to our enemies”, meaning against our enemies. They will act as if they too wish to “fight amongst us”, and leave the country with us to fight. Then, when they will be outside the country, “they will depart from the land”, and this is our goal.