The IRS would already know exactly how much money she made on Only Fans in any given calendar year, due to the Form 1099 filing requirements that exist in the US (there is nothing similar to that in Canada). The IRS gets a 1099 form from OnlyFans, which would disclose the earnings by every US resident who is being paid from the platform. There could be a middle man in charge of her Only Fans account who is getting paid -- that middle man, who would certainly be on the IRS radar because of the $size, would then be required to fill 1099s for anyone they issued payments to.
In Canada there are generally only tax slips issued by employers and for certain investment income. In the US if you do any consultant work or services for a company, make any money / receive payments from a digital platform (such as Only Fans), make money over "x" amount from a gambling site, those companies who pay you are required to submit 1099 forms to the IRS. So she can't hide it.
All that being said, while I'm sure she is making several million from Only Fans, I assume the $43million is fake and is a social media power play to try to generate more attention for her.