The information you were given is not correct. If you stop and think about what they told you, it would not even make any logical sense. If this money was not taxable to begin with, then what would it have to do with whether or not you earned income
Even though you are not a US citizen, and have no earned income here in the US, you have received income from a US source, and that is taxable to anyone who receives it. That is exactly why they withhold these taxes at the full tax rate
of 30%. When someone who is not from the US wins a prize or is paid other money, the IRS
would never have any assurance of collecting tax on that money unless they withheld the tax at the time of payment. Because not many people who were not citizens of this country would ever end up filing
a return and actually paying the tax.
Even people from another country who do business with companies here in the US must have 30% withheld from their payments by the US company. It is the IRS law
I hate to give you news that you probably did not want to hear, but any money you receive from a US source is subject to 30% federal tax, and the applicable state tax, which in California is 7%.