Not exactly.
If you ONLY have party tickets and no other tickets at all, then you can get FP+ for the 4:00-6:00 time frame, and no one in this situation has reported getting an email that they didn't have tickets.
If you have party tickets and you have tickets for other days, it depends. If you're going to MK on your regular ticket on the same day you're going to the party, you still get your FP+ just as if you weren't going to the party. If you go to another park on the same day as your party day, you can get FP+ at the other park, but you can't get FP+ for the 4-6 pm at MK before the party or you forego FP+ at the other park and make FP+ for 4-6 pm at MK. If you aren't going to ANY park on the day you're doing the party, the system will allow you to book FP+ from 4-6 pm at MK (most of the time), BUT some people in this scenario are consequently getting emails saying they have more days FP+ booked than they have tickets to do so. Example: you have 5-day tickets and a party ticket and you make FP+ for all 5 days plus your party day. They system now thinks you've made FP+ for 6 days (which you have), but it only sees the 5-day ticket.