I have to disagree with the OP about 'it is said clearly that their system is not democracy' - Heinlein argued against the 'unlimited democracies of the 20th century', where people were granted the franchise of voting without any effort or sacrifice given to it by the people (simply given when you come to age). Thus no true value was held for the ability and right to vote.
In the book, you were a civilian until you passed the rigor to become a citizen. If I remember right, 2 years of service needed. Anybody could become a citizen, even if you were without limbs, or dumb as a rock, etc. You would be given some role, even if it was not prestigious (like experimental medical test subject). As long as you were deemed capable of understanding what you signing up for. The important thing is that you committed yourself in service of something greater than your mere self - society. Commitment to the greater whole of humanity.
And a key function is that if you chose to not go through with this and remain a civilian, you were provided the same rights and protections as any other - bar the franchise of voting.
I would argue this version is a more enhanced vision of democracy, rather than considered oligarchic in any consideration.
Oligarchy is restricted to a few elites that are considered above and responsible for key parts of a nation (could be military, economic, or stratified as nobles, religious leaders, etc.). The democracy and right to vote here is available to anyone, and equal in nature. The population percent that can vote is only restricted by who wishes to enlist.
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For fun reading. And free book link if anyone wants to read.
https://wrathoftheawakenedsaxon.files.wordpress.com/2016/09/starship_troopers_-_robert_heinlein.pdf
"Now here are we with still another system... and our system works quite well. Many complain but none
rebel; personal freedom for all is greatest in history, laws are few, taxes are low, living standards are as
high as productivity permits, crime is at its lowest ebb. Why? Not because our voters are smarter than
other people; we’ve disposed of that argument. Mr. Tammany can you tell us why our system works
better than any used by our ancestors?"
I don’t know where Clyde Tammany got his name; I’d take him for a Hindu. He answered, "Uh, I’d
venture to guess that it’s because the electors are a small group who know that the decisions are up to
them... so they study the issues."
"No guessing, please; this is exact science. And your guess is wrong. The ruling nobles of many another
system were a small group fully aware of their grave power. Furthermore, our franchised citizens are not
everywhere a small fraction; you know or should know that the percentage of citizens among adults
ranges from over eighty per cent on Iskander to less than three per cent in some Terran nations yet
government is much the same everywhere. Nor are the voters picked men; they bring no special wisdom,
talent, or training to their sovereign tasks. So what difference is there between our voters and wielders of
franchise in the past? We have had enough guesses; I’ll state the obvious: Under our system every voter
and officeholder is a man who has demonstrated through voluntary and difficult service that he places the
welfare of the group ahead of personal advantage."
"And that is the one practical difference."
