Definitely looks confusing but it’s actually pretty simple. The rationale is that interest net of taxes is accounted for above the line (i.e. within Net Income), so you’re adding that amount back in to calculate FCFF essentially as a contra entry. Effectively if leverage increases Int(1-t) will increase, however there will be an offsetting reduction in Net Income. Does that make sense?
Fundamentally leverage will not affect FCFF as it is a measure of the FCF available to all providers of capital. Adjusting the debt/equity mix is essentially irrelevant with respect to the overall quantum of FCF, rather it simply affects the allocation of FCF between equity and debt providers.
Changes in leverage will however have an effect on FCFE, as positive Net Borrowing will increase the amount of cash available to equity holders. All things being equal, subsequent periods should see a negative impact on FCFE relating to the increased interest repayments required to service the higher debt load.