In my opinion such BTC stockpiling can be dangerous like a 51% attack but I didn't find any information is Satoshi Nakomoto ever mentioned that.
Not at all.
that attack doesn't work in this case as explained above. And even them stockpiling almost 3% (506000) of the current supply (18.5M) is kind of a nonissue considering we're talking about an investment company rather than a single or several anonymous whales
hodling the coins, now that would be worrisome cuz you'll never know if they're try to crash the market or whatever.
As long as Coinbase Custory
[1] are keeping the coins safe, it's all good.
1. https://www.globenewswire.com/news-release/2019/08/02/1896358/0/en/Grayscale-Selects-Coinbase-Custody-as-Custodian.html