The classification of anyone that beats the market as a lucky unicorn is logically impermeable. If you accept the argument, then no matter who says they're beating the market, you'll be able to dismiss them without further thought.
There needs to be a better argument, one that allows for the hypothetical fund that does manage to do something intelligent and beat the market.
I find that this style of logically impermeable arguments crop up quite often when dealing with peoples belief structures. Here yours is that the stock market cannot be beaten, which may or may not be true. A useful question to ask in these situations is; Suppose that your belief is wrong, what evidence do you imagine might prove that it is in fact wrong?
When you can answer that question, you'll have a more concrete argument for why this fund is indeed just a lucky survivor.