They have it right. You're underdeveloping it, so if you develop it further and it's too dark, that means you're overexposing it.

Even though the recommended development time for Muligrade 1+9 is 60 seconds with RC paper, I always go 90 seconds (except for contact sheets, when I'm more interested in speed of the process). Even though the image doesn't obviously change, you get slightly denser blacks and slightly more highlight detail with the extra development.

If and when you graduate to fibre paper, the development time will be even longer.