What does this mean for the security of the testnet?
Aside from what other member say, it means Signet is better choice for some people.
Quote from: https://en.bitcoin.it/wiki/Signet
- You want to test long-term running software and don't want to deal with tens of thousands of block reorgs, or days of no blocks being mined, as is the case with Testnet.
- You want an easy way to test double spends (signet plans to include support for automated double spends, where you provide two conflicting transactions and they are mined in order, with a reorg happening between them).
The only folks who would get hurt are the assholes that 'mine' it for-profit by getting tbtc & then selling/exchanging it for other coins and the idjits that buy them. Seeing their 'holdings' disrupted certainly would not break my heart 
Not if the 51% attacker mine empty block, which prevent everyone TX from getting confirmed.