The rule you are quoting is not from the current rules – please refer to the 2017 version of the rules: https://rules.wfdf.org/rules/2017/17-11-out-of-bounds
This is allowed:
As most of the players (imho) I put one foot into the field (estabilishing the pivot point in-bounds) while the second food still remains out-of-bounds and I put the disc into play.