2 Wraiths and 6 Specters is correct
1. Three risen dead don't turn into a single specter
The wraith ability "Create Specter" specifically creates one specter per one dead humanoid. While you can rule as you wish, for me personally it is intuitive that souls are on a 1-to-1 basis. There's never been a case of a "fused" ghost that I can remember.
2. The room is big enough that it makes sense
As you can tell by reading the legend, the room is actually represented by tiles of 10 square feet rather than 5, meaning the treasure room is a 30ft-square (or a 6x6 grid, if you prefer). 8 monsters and some PCs is just fine. As an addendum I should remind that this is actually just Diderius' bedroom so it's fine if it's cramped for so many people to fight in it.
3. This room was so deadly that the entire cultist group failed to clear it
The cultists went through other doors and fought undead guardians but didn't mark anything so clearly and adamantly as this one, with "DANGER" written on it and spiking it shut.
They were so thoroughly terrified by the slaughter that they left a retinue of devils on watch duty just in case the undead guardians burst out.
Story-wise, this room was deadly to a party much larger than the adventurers' one, and it would be dishonest to dial it down (or fudge if you will) for the sake of PCs. This room is quite literally marked for the PCs not to go into it, and they should pay the price if they ignore the warning.
4. This room is not vital to the dungeon
They can skip this room entirely, it's more of a bonus challenge with bonus rewards. Even if they go in, they can leave if they don't want to fight the pack of CR 1 creatures. Apparently the undead didn't follow the cultists out.
5. Encounter difficulty calculations are wildly inaccurate
While you may think the encounter is beyond deadly due to XP costs, you'd be surprised at how effective player characters are. A single mummy lord encounter is even deadlier than 2 wraiths + 6 specters, XP-wise, but you'll find he may not even get to play his turn before he dies. Specters are CR 1 enemies and can be taken out in their entirety by a couple of fireballs. The undead horde may be crippled by an averagely successful Turn Undead. Nothing about the composition of the undead suggests this encounter is actually hard.
6. The encounter is more than just the undead
It seems to me that the author knew how underwhelming the encounter is to the PCs (combat-wise), that he explicitly reminds us
to have the devils attack the party when they come up the stairs, completing the combat portion of this encounter.