yes is questionable practices no doubt with Coinbase supposed to be the most compliant with regulations and having insured funds.

what most gives credence to this story is the moving of funds between so many accounts on Poloniex and new accounts from Coinbase with portion returning to Poloniex again.

also the users who tweeted this are experienced long term crypto people,
i know cryptocobain really goes off sometimes but has been correct on multiple occasions mt gox mintpal cryptsy to name a few.

If it was known precisely how is happening Im sure many would be doing the same,
only ones who would know for sure are would be dev teams at Coinbase and Poloniex

I dont know what kind of explanation you would like,
The txids and wallet address are there for review also the tweets i found relating to the story.