Yes it is. 5 levels and enough new missions to get those 5 levels IS an expansion. Its a small expansion, which is why its cheap.
A new large planet in SWTOR is equivalent to a handful of zones in other MMOS, which is what is considered an expansion if delivered all at one.
That is still an adventure pack though. Expansions have more than a handfull of zones, or have a handful of zones the size OF Belsavis.