I have been around MN hockey my entire life and never heard anyone say this. Zero to it.
The December 05 bdays started playing hockey with there May 06 buddies and have played the same amount of hockey growing up. There is no advantage to being a late bday in Minnesota.
It's actually harder for late bdays because they need to make NTDP after their 9th grade year which is tough, if they want to go that route. And they have to play selects against many kids a grade up.
Being a late(r) birthday means you're one of the oldest in your school year grouping in the States. While a US born October 2007 would be disadvantaged competing in birthyear minor hockey against January and February 2007s, they would be amongst the oldest in the class of 2026, whereas a May 2008 would be relatively older for the 2008 birthyear for minor hockey, but the youngest in the class of 2026. Much different situation than in Canada.
If the December 2005s are playing bantam minor hockey with the May 2006s, then there would be a developmental advantage for those 2005s like Moore by simply being older.
I will note that what I bolded in your comment is very true.