The person with GCSEs gets a 5 year head start, so even if it takes them a while to land a job, and then 3 years to skill up, they could still come out ahead. As you say, the graduates struggle to get entry level jobs, so it's not like all of them get to 'jump the queue' and walk into an elite job at 21. I certainly didn't, would have been ahead for sure going in with GCSEs or A-levels.
Tbh, the main benefit of my degree was it allowed me to study a Masters via the OU later on when I was older, that avenue wouldn't have been open to me to be fair.