An incognito kill does count as a silent kill, so even if you get 5 incognitos in a row you should still get the killstreak. The way it works is that if you have silent or higher, you get a certain "silent bonus" (200 points) score when you get the kill. That's what counts it as a silent kill. If you got an incognito kill, you get the "silent bonus" as part of the "incognito bonus" (300 points) so it still counts.
Also, here is a reference that says:
- Streak Bonus +300: Awarded for killing three suckers while Silent (or Incognito), this bonus doubles the value of your kills if you can make them stealthily. In general, I recommend this Kill Streak for cautious, patient play. It’s easily doable and can add thousands of points to your score every game if played right.
- Streak Bonus +750: Murder five people stealthily and you’ll be awarded 750 bonus points! Though this Kill Streak offers the greatest per-kill bonus, it’s also exceedingly hard to accomplish. In Wanted, Assassinate, and Alliance, odds are you will be taken out before filling five body bags. In Manhunt, Chest Capture, and Escort, you will rarely have the opportunity to remain Incognito while also being effective. Stick with the +250 or +300 bonuses instead.