This pre-supposes that doing anything pro-applicant is somehow a benefit to a company. Many of the things you've said are a benefit for the company, but many are not and are instead cultural.
"It's to a company's benefit to miss twenty qualified applicants than to hire a single unqualified one." This one is true, but not universally. The assumption baked in is: Interviewing more candidates is less expensive than hiring an unfit candidate and then letting them go. That's usually true, which is why this piece of advice is brought up so frequently.
"It's to a company's benefit to miss twenty qualified applicants than to reveal their salary range during negotiation to a single one." The assumption baked in is: You "the company" will save money by never revealing salary info; by revealing salary range you're giving up dollars you could have negotiated for with the candidate. The money spent interviewing candidates who you can never hire is less than the money you'll save through the lower salaries you can negotiate for because you don't reveal pay info. This is also mostly true, since you can save so much over time through negotiating a lower salary.
"It's to a company's benefit to lose twenty qualified employees than to promote anyone from within." The baked in assumption is: Promoting from within is more expensive or yields worse results than hiring externally. In general, this is usually not true to a point. I'm not saying hiring a factory line worker to be the new CEO is a good "hire from within" practice. Hiring from within when there are jobs based around similar skill sets but variations in focus area/responsibility level is probably a good idea. After all, why spend a lot of money looking outside for someone who is good at X when you already know a load of people who are good at W or Y, which are very close to X. It's the same reason that referral programs are such good ways of finding candidates: it acts as a fantastic filter. The reason it's often not done is largely because skill gaps (or perceived skill gaps) are too big for a role change.