Yes. It's a significant margin - and given that the other 60% of coaches come from a pool that includes literally every other school in creation, and you can see that there's a serious preference for alums. And its an understandable preference.
Being an alumnus does not mean you're going to be a good coach, no. But it's certainly a bonus from a number of standpoints, including marketing. You don't think a dispirited fan base would be inspired and turn out to see the favored son return? Especially when he's one of the most sought after coaches in the entire country? Riga or Gosek both have impressive CVs, but they'll never have that kind of instant connection. Is it an intangible? Yes. But you're dismissing it out of hand.
How many have winning records? Well, Jerry York (BC '67) is the all-time winningest coach in college hockey history. Jack Parker (BU '68) is third. Red Berenson (Michigan '62) is fourth. Rick Comley (LSSU '72) is fifth, he started out his D-I career at his alma mater for three years - in fact, taking over just two seasons after he was playing. Len Ceglarski and Jeff Sauer are 9th and 10th all time, both coached at their alma maters. That's 60% of the top 10.
Ted Donato (Harvard '91) is in the Frozen Four with his alma mater this year. Norm Bazin (UML '94) and David Quinn (BU '89) were both one goal away from doing the same with their alma maters - which both have already done in the recent past. Enrico Blasi (Miami '94) and Bob Motzko (SCSU '87) have also taken their schools to the Frozen Four recently. Keith Allain (Yale '80) and Brad Berry (UND '02) have won recent national championships with the same school they played at - as has York. Been a while for Berenson too, but Michigan has an all-time record for consecutive NCAA appearances under his reign.
I can go on.
This is really reaching. I'm sorry, but there are a whole host of successful coaches whose first head coaching experience was in Division I.