Welcome to the real world.
... is condescending, since it implies the person you're talking to has been living in a fantasy world up to now and is ignorant to how the (real) world works (and it's entirely non-constructive, since just explaining how the world works already implies that they don't know how it works).
This may be less offensive if the asker is just entering the working world (which I didn't read in the question), in which case it may be more of an "maybe it sucks, but this first experience of yours is pretty common" (which is still not particularly constructive though, since that meaning may not be obvious and this point has been stated more explicitly later on - "that's not how the industry works" - and repeating oneself is generally also a bit condescending).
Sometimes you have to work with other people ...
... is so obvious that it's condescending.
If you're a Free Software zealot
... is (moderately) offensive, since "zealot" could be seen as having a negative connotation due to "zealots" commonly using offensive or violent means to further their cause.
Your entire post at the moment screams "high maintenance, unwilling to make reasonable compromises"
... is basically an insult (if you want to say some specific things, like refusing to provide a Word resume, is bad or would be seen as bad by someone, that could be good and useful advice, but calling everything someone said bad is just insulting and not constructive).
have a serious think
... is also (a bit) condescending, since simply noting the ultimate point of the comment ("this is not the attitude you want to project when looking for a job") or phrasing it in terms of a question ("is that the attitude you want to project when looking for a job?") would already lead to them thinking about it, so you're kind of saying they're not smart enough to think about if not told to think about it. Adding "serious" to that implies that they're not smart enough to figure out what deserves how much thought by themselves and need to be told that.
On the offensotometer, I'd give it a 3/10 - not that offensive, but not not offensive - probably fine for Reddit or Quora, but Stack Exchange should have higher standards, especially on a site where professionalism is our most used tag.