I disagree with this statement.
From what I saw, how the network behaves. I think that due the low block time, the network cannot distibute the mined block fast enough and we get orphans.
Even with a high hahsrate, the network will always have a big amount of orphans due to the low block time.
I believe, that in order to make Grain stable, we need a block time of at least 60 seconds.
There is a good reason, why the major coins have a such high block time and not 30 seconds or less.
Grain has now a block time of 15-25 seconds, this cant be stable.