As the 2nd tweet explains, the difficulty of Ethereum changes dynamically amd can decrease the longer that a block hasnt been mined, this is not the case in Bitcoin.
Bitcoin also adjust difficulty - but does that every two weeks or so. would it make sense to mine for a sibling for the last block before the difficulty increase while everyone else mines for a child block with increased difficulty? thus having twice the relative hashrate for 10 min every time difficulty increases.
No, this attack wouldn't work on bitcoin. Bitcoin chooses the chain with the most total work (which is blocks x difficulty), not the most blocks. Else someone could fork the chain from a block sometime in 2009 when difficulty was very low and outpace the rest of the network.
This is not true. Bitcoin has a difficulty adjustment.