Annexation wouldn't support the picture of a continuous Polish nation stretching from the early beginnings, over the time of the partitions to modern day Poland.
I don't see why not - if you look at the Yugoslav idea of the constituent people also being a nation, then they don't have any break in their history. When you consider all the events between 1795 and 1918, it's obvious that the idea of the Polish nation was still more than alive even if they didn't have control of a territory of their own.
Certainly, the Polish people felt they were under occupation, but the accepted legal view of things is that they weren't.
Wikipedia perhaps has the best definition :
Military occupation is effective provisional control of a certain power over a territory which is not under the formal sovereignty of that entity, without the volition of the actual sovereign. The intended temporary nature of occupation, when no claim for permanent sovereignty is made by the occupying entity, distinguishes occupation from annexation.
I think it's also worth questioning what the "Polish" people actually were at that time. I know in the early days of the II RP, people often identified with their local area far more than they did with the idea of Poland - and I think it was the same story in areas such as Galicia under the Austrian administration? Of course, there was a huge Polish minority in the three areas - but how many people were classed as Poles when they actually weren't?
It is a curious footnote in history to see how there were Polonisation campaigns during the Austrian administration.