For the DMG method, I was choosing whatever CR would net a medium encounter for 4 PCs with 4 of them, whatever CR worked for between the listed Medium and Hard XP.
Well there's your problem. If you're trying to see whether the UA encounter guidelines incorporate multipliers for increasing numbers of monsters, you have to, you know, examine increasing numbers of monsters. Don't fix it at 1 monster per PC. Checking lots of levels doesn't tell you anything.
Let's pick level 10 PCs arbitrarily (because I'm not going to analyze every single level just for an Internet post), and look at various numbers of monsters and what implied multiplier there is for each.
CR 1/8, 12 per PC, so 48 giant rats. Kobold.com says this is 4800 XP (right on the 4800 threshold for Medium). If anything, this is using too large a multiplier, because it's not a Hard fight.
CR 1/4, 12 per PC, so 48 goblins. Kobold.com says this is 9600 XP (over the 7600 Hard threshold but not up to the 11,200 Deadly threshold). This seems to be using approximately the same multiplier (x4) as the DMG.
CR 1/2, 10 per PC, so 40 orcs. Kobold.com says this is 16,000 XP (Deadly), so the multiplier here seems to be a bit low, maybe only x2. It's underestimating the orcs. (If it weren't using a multiplier at all it would be calling 40 orcs Easy, and it would be asking instead for 76 orcs to hit the Hard threshold. So it seems to be using a x2 multiplier to be satisfied with only 40 orcs.)
CR 1, 5 per PC, so 20 giant hyenas. Kobold.com says this is 16,000 XP (still Deadly), so again the multiplier is a bit low, around x2.
CR 2, 2 per PC, so 8 githzerai. Kobold.com says this is 9600 XP, Hard, so the implied multiplier is once again in line with the DMG (about x2).
CR 3, 1 per PC, so 4 githyanki. Kobold.com says this is 5600 XP, Medium, so the implied multiplier here is higher than the DMG, maybe x2.5?
CR 4, 1 per PC, so 4 chuuls. Kobold.com says this is 8800 XP, Hard, so the implied multiplier here is in line with the DMG.
CR 5, 1 per 2 PCs, so 2 drow elite warriors. Kobold.com says this is 5600 XP, Medium, so the implied multiplier is too high here, maybe x2. UA is overestimating the drow.
CR 6, 1 per 2 PCs, so 2 young white dragons. Kobold.com says this is 6900 XP, Medium, but that's pretty close to the 7600 Hard threshold so I'll give it a pass and say it's reasonably close to the DMG.
CR 7, 1 per 3 PCs, is not evenly divisible for a 4-PC party, but for a 3 PC party 1 Young Black Dragon would be 2900 XP (Easy, where Medium is 3600), so the multiplier is way too high here.
CR 8, 1 per 4 PCs, so 1 young green dragon. 3900 XP, Easy (short of the 4800 XP Medium threshold), so the implied multiplier is again too high.
I won't bother with CR 9 and 10 because the table says you need 5 and 6 PCs for that.
As you can see, the UA values for a four-person party are all over the place (they are clearly derived from the DMG tables using a party size of one, not four), but the theory that multipliers are not used at all is not born out by the data. If anything it is the small fights that the UA gets wrong.
