1. it seems to me that you misunderstand the point of luukas's comments. he doesn't seem to "support" gatekeeping materials, clearly not as he is actively contributing on this forum by providing resources and assisting with questions. he is simply stating that students are often put into situations where they feel like gatekeeping materials is necessary for success due to the pressures of ranking and competitiveness within many high schools (notably selective ones, and you cannot argue that rankings are not a competitve system that does have a major impact on results), and that in some sense it makes sense why students would keep resources to themselves, as it does literally provide them with a major advantage in the hsc and atar systems, which lets face it is an incredibly competitive race where you try to finish ahead of the rest of the state/country. he even goes on to compliment those who continue to share despite these pressures.
2. like it or not, students HAVE to compete in order to make it into whatever tertiary course they desire. you are describing a perfect world here where teachers, peers, textbook writers, historical contributors to subjects etc are all credited with helping progress a students "individual achievement" and everyone wins; but this does not exist. a uni looks at the score that YOU have recieved, and how competitive you are relative to other students, including those from your cohort who you are competing against. unfortunately, they could really care less about the "systemic achievement" and "economic model" that you are discussing. in reality high school students are placed in a situation where they must try to beat everyone else, this is why parents will pay for tutoring to try to make their child get ahead of others at their school. naturally, students are inclined to gatekeep their resources, as it literally provides them a systemic advantage which may be the difference between their access to their dream course or not. if this perfect world was true, then no student would gatekeep their resources, as there would be no incentive to do so, but unfortunately this is not the world we live in.
3. you are being quite dismissive of socioeconomic boundaries that exist. not every student has access to external tutors such as yourself, not every student can afford to go to a private school, or afford to move near a high ranking school if they are rural. naturally, students who are disadvantaged are now placed in a situation where they are fighting against students who are receiving huge amounts of support. these students already have a lack of resources compared to these more advantaged students, and due to the highly competitive environment of the hsc feel as though they have no choice but to gatekeep their resources. on top of this the advantaged students also want to remain advantaged, and so will not share their resources with those around them. this is a socioeconomic boundary, like it or not this exists and is prevalent in systems where you must compete amognst one another such as the hsc.
your view that everyone shouldn't try to beat each other in the hsc is literally the exact opposite of the point of the hsc/atar system. this system is built for students to be ranked amongst one another (australian tertiary admissions rank), which naturally encourages students to gatekeep their resources, as they feel as though this is one way they can climb the ranks. of course this is not the way a perfect system operates where everyone can cooperate and achieve their best together by sharing their resources, as has been said these systems do exist to an extent such as at university often students are more collaborative as rankings are less important than just pure grades. but the reasons why students gatekeep their resources is much deeper than "right vs wrong", it could be from socioeconomic boundaries, competitiveness for certain tertiary courses, or simply just the pressures placed upon them due to the environment of the hsc or certain schools. it is very dismissive to call all the students who gatekeep their resources "overzealous", many (if not all) of them are just playing the system they are put into.