No, what this guy is saying is total bullshit. I have worked in college admissions before and emphasis is put on GPA as much if not more. Seriously, test scores show potential but GPA shows motivation. Schools these days are looking for motivated individuals with potential.
To make this running related, let's put it this way: You're recruiting high school students to your top tier DI college: You have guys running 4:10 for the 1600 and a guy who runs a 4:15 for a 1600 but the guy running 4:15 has only run 20 miles a week but comes from a good high school program. Sure, he's only run 20 miles a week, and sure, he's got great potential, but the fact that during high school he never showed up to practice even though he could have tells you as the coach a lot about him. Maybe if you gave him a scholarship, maybe he would start committing to the team more, but chances are that when you tell him he's gonna have to run 60, 70 miles or more a week and put in a good commitment to run with the best, his record will probably crack. It's not that he was stuck in a shitty high school program that trained him poorly, it's that he clearly didn't have the commitment or dedication. If this was the only guy you got, sure you might take a chance on him, but why not taking another 4:10 guy who is guaranteed to be totally devoted?
Your GPA says a lot about you. It's not just about how hard you worked one year, but for four solid years. If you can't prove you're committed to working in high school but had great potential, colleges will question your motivation at their school.