This has nothing to do with Hivemind in particular, but effects all Web3 DApps and even Wallet Apps on your desktop or smartphone. If you use etherscan.io for example, it accesses a centralized db and could theoretically return false or manipulated data. This risk is mitigated by decentralization. You can switch to other block explorers or Hivemind nodes and compare the results, or even run your own full node + Hivemind if you wish so.
RE: Layer 2 is not layer 2 without layer 1 security