If someone attacks my Rift Herald and wins the combat, when I use the Rift Herald's Death Knell to play the unit from my hand to that battlefield, do they score a point before the new combat begins?

No, your opponent does not score a point before the new combat begins.

Here is the breakdown of why this happens:

1. Timing of Deathknell

When your Rift Herald is killed in combat, its Deathknell ability triggers. Per Rule 808.1.d.2, this trigger is added to the chain as a pending item. The combat showdown is currently ongoing; therefore, the chain is active and the game is in a "Closed State" for the purpose of the showdown.

2. Resolution of the Chain

When the Deathknell effect resolves, you play your unit. While this unit enters the battlefield, the showdown (and the combat) is still considered ongoing. Control of the battlefield is not established until the showdown fully closes (when both players pass priority in an Open State and the resolution of the combat is finalized).

3. Combat and Scoring

Because your new unit has entered the battlefield during the active combat, it is now part of that combat. The rules for scoring (Conquer) require the showdown to conclude and a player to establish control of the battlefield by having the only remaining units (or by being the winner of the combat).

According to Rule 461.3.d.1, if there is "No Result" in a combat because units from both players remain present, the combat is simply staged to continue at that battlefield. Your opponent cannot conquer or score the battlefield while they are still locked in combat with your newly played unit.

This is my interpretation based on the combat showdown rules and the trigger timing of Deathknell. No official FAQ exists for this specific timing interaction.