You’re not running down Helvellyn, it’s more the cumulative downhill that qualifies a “downhill” marathon. Remember this is 1500 feet drop over the course of 26 miles, so you’ll get the benefit of gravity without the downsides of steep drops.

I ran a downhill marathon that was on a rail trail through the mountains. It was a very very gradual downhill the whole way.

My friend that ran it was mad at me when we finished because he thought parts of it were uphill (even though it wasn't.)

That race won't be a Boston qualifier next year.