Secure from malicious app programmers (Unlike what other people think open source doesn’t equate safe, even reputable essential ones can be malicious like recent xz one)
Secure from remote attacks and botnets (Only reason this haven’t been a larger problem is because linux desktop users are too few to worth targeting, though that may change with rise of steam deck)
Physical integrity guarantee and protection against manufacturer while nice are very hard to get in current climate
proper sandboxing and permissions, auditable code and small attack surface as opposed to spaghetti code glued together that’s impossible to audit, regardless of threat model those things are needed, even linux is moving in that direction (Though very slowly and very half baked like with flatpaks)
While that’s true for mitigations, one system can be more secure than another by design
Things like an OS that’s designed with sandboxing, more clean codebase that’s auditable, permissions, … in mind is more secure than an OS that later adds them as an afterthought
Or at least if added later they should be done properly
iOS and Android are way more secure than Linux (And no Android isn’t just linux) cause they were designed in much later era with better security practices in mind
Even MacOS and Windows’s security are objectively better than linux’s even though they started with same security mindset, the problem is they are not open source