If you read carefully, that isn't what it says.
It says:
a) At the start of February, there was an increase in incomplete transfers, which caused them to investigate
b) When they investigated, they found that 850,000 coins were missing
They don't say that the 850,000 coins were transferred in February, just that that was when the first noticed that coins were being transferred. The transfers could have been happening for months or years before that without them noticing.
If you choose to believe what MtGox says as truth, you can also consider true their claims that they were "AT ALL TIMES" accounting for BTC and FIAT balances.
Hmmmm, no, you CAN'T consider them both to be true, its logically IMPOSSIBLE.
Anyway... We are still waiting for them to report at which exact time the balace discrepancy started. That's very easy to prove forensically matching internal accounting data with blockchain ledger.