Because according to both
fastboot help and online documentation for fastboot the fastboot boot command is intended to
Download and boot kernel from RAM.
No mention there of using it to temporarily boot a ROM image. That may be possible / necessary for devices which have no recovery partition, but that is not the case for the Z3C
In this post a user has followed those instructions, and his device will not boot into recovery.
I will try for myself later on, and let you know the results