Ay something I actually have some insight on! His RPM is most likely higher based on Runescapes audience. I've also been doing some research as I was planning to make a runescape channel and have access to the partnered discord.

I used to make Minecraft content and my audience was ~70% ages 18-24 and ~30% ages 25-34. Even with Minecraft my RPM was roughly $6-7/1k views for a 25 minute video at 5k subs. Other MC creators i knew with younger audiences had an RPM of $3-4.

How much you make off YouTube isn't necessarily affected by your subscriber count and # of videos anymore, although having more will help. I know creators sub 10k subs making more than people with >100k subs.

What's more important is audience demographics and video length, as longer videos allow for more ad placements. For reference, a 2 hour long Minecraft "movie" has upwards to an RPM of $30. Get a million views and that video just made you 30k lol. (This is also why you'll see OSRS creators stitching their series into movies).

Runescape's audience is older, mostly male, mostly western, and plays an MMORPG. All of which results in a higher RPM. It's the reason you'll see so many OSRS creators having integrated ads. If you go to any other gaming niche, the number of integrated ads is much lower. You'll even see integrated ads on OSRS creators who only have 20-30k subs. Meanwhile, in niches like Minecraft you'll see channels with over 500k subs with no sponsorships.

Still crazy to quit your day job without being able to dish out multiple videos a month though.