Anyone that reads the LN Whitepaper , can see stealing BTC from LN, requires nothing more than evading their time locks.
That is true for any 'refundable' time locked txn, like Atomic Cross Chain Transfers.
If I can stop you from claiming the coins before the time lock expires, i can refund them back to myself (after I have taken your coins aswell that is).. Just have to prevent your TXN from getting on chain.
This is
much harder to do than I think you are implying.