Just did this to my T-Mobile SGS4. It worked just as described. The Knox counter wasn't tripped, but Custom is indicated on the Download screen (Vol Down+Home+Power).
Here's what my Download screen now says:
PRODUCT NAME: SGH-M919
CURRENT BINARY: Samsung Official
SYSTEM STATUS: Custom
KNOX KERNEL LOCK: 0x0
KNOX WARRANTY VOID: 0x0
WRITE PROTECTION: Enable
eMMC BURST MODE: enabled
Thanks for the link to the other thread.
Now I can use Titanium Backup, Greenify, ADB - rooted, esFile Explorer - rooted. Not needing a Custom Rom or Custom Recovery yet. And, to get it back to un-rooted, there are instructions in the other thread about that.