If WACC decreases, NPV increases. Whether or not the NPV is positive depends on whether WACC is greater or less than the IRR. If the IRR exceeds (is less than) the WACC, NPV is positive (negative). If the WACC decreased, but is still greater than the IRR, the NPV is still negative. It is just less negative.
Yourstruly isn’t really wrong. The IRR computation is completely independent of the WACC, you just need WACC to determine whether or not to accept the project. IRR doesn’t change when you change the WACC, but your decision may change on whether to accept the project or not.