I've seen many managers fail for this reason.
Every time I read or hear Gartner Group "information" piece on a subject that I am competent to judge the value and accuracy of the information, it is at best useless fluff, typically is mostly wrong, and commonly totally wrong.
"An average of 10 reports" goes against the commonly accepted magical number[1] of 7 +/-2. I suppose if the "pure" managers are only pushing paper (unable to judge technical merit), they can support more direct reports. That structure sounds like a recipe for perpetual technical problems.
What you quote sounds like classic matrix management[2] which results in two bosses, one controlling your HR side and one controlling your technical side. That does not work well in my experience because the two managers have very conflicting goals and neither of the goals are in in alignment with your goals. In the ensuing fight, the HR manager always wins because he controls your pay and assignments... and usually is the manager whose goals are least aligned with your goals.[3]
[1] https://en.wikipedia.org/wiki/The_Magical_Number_Seven,_Plus...
[2] https://en.wikipedia.org/wiki/Matrix_management
[3] 24 years 9 months in a large corporation with matrix management. It was OK until the Ginormous Enterprise bought the division and sucked all the fun (technical work) out of it.
1. Yes some people are fine with less, but this is the average amount of time people need. Putting in less just results in a build up of management debt that will come back and bite you in the butt.