For a series of even 20 cash flows from a non compounding loan payments (p&I)every two weeks, xirr is giving very different results then IRR*20

Is xirr giving the effective yield which is incorrect here because it's simple interest?

The conversion is not XIRR = IRR * 20. It's XIRR = (1+IRR)^(365/14). The fact that it's 20 cash flows is irrelevant - the fact that each period is 14 days is relevant.

Not sure what how simple interest enters into this. XIRR gives the annualized effective yield, which is a compound interest value. IRR returns the yield per period. Unlike XIRR, IRR has no way of knowing the length of each period, and therefore has to solve for the per period value, with no way of converting it to annualized.