For me, zero running gives sub 20, running or biking 1-2 times per week gives sub 19, and base training for sub 17.
This probably scales for better and worse athletes. if he has talent, breaking 18 but not 17 probably falls within "training but not even serious enough to call it a solid base phase," which is common for HS team members.
If you are a 13 minute runner running sub 17 is still harder dan sub 18.
So, what is the question?
I find that running 17:30 is noticeably easier than either 17:00 or 18:00. Running too slowly hurts my knees. Running too fast makes me tired.
For me, with a lifetime 5K PR of 16:47, yes it was. My PR was stuck at 17:01 for about a decade.
If you are running your hardest, the effort level is the same no matter what the clock says. As you get better, It doesn't get any easier, It just gets faster.
One can train down to 17, usually. The adjective noticeably is vague. It could mean more mileage, intensity etc. But everyone knows that. OP doesn't sound like a runner.
I partly disagree, but also do agree, the first race of this season I had was a 19:50.99 5k. And it was definitely a slower course, but then the next race 2 weeks later I topped at 18:43.49. But I agree in the case of, most people it can be harder, but it really all depends.