So he took in 300k... how much has he paid out? Without a reference point, coins flowing in means nothing.
I think the 300k is a rough estimate of current holdings based on last week's interest payment.
It appears that at least 22k BTC was paid out in interest last week. Assuming nobody automatically re-invested, and that everyone gets 7% per week, that means total holding are 22k/0.07 = 314k BTC. Of course some people do re-invest, and some get lower interest rates, so the real figure will be higher than this.
How do you account for the people that ask for withdrawals timed with the interest payments? That will artificially inflate their interest payment by a factor of 7.
Do regular withdrawals appear in the same transaction as interest payments?
People have a singular withdrawal address. If you request a withdrawal, it appears just like an interest payment.