Let me clarify, that may be the intent, but it is not how it is working. From their examples:
"From the conversion table, we see that this CR 11 remorhaz may be used in place of a ML 11 Solo, a ML 16 Elite, a ML 20 Standard, or a ML 28 Minion."
A CR 11 remorhaz is not equivalent to one lvl 20 PC. That is my point. It is not far off, IMO, but not equal.
A L 20 fighter, L 20 wizard, and L 20 rogue are not equal. So there is no set of stats that is equal to a level 20 PC, as there is more than one level 20 PC.
They are not far off, but are not equal.
And the CR 11 remorhaz is about as close to those level 20 PCs as the level 20 PCs are to each other. You know, depending on build.
The Remorhaz does about 70 damage per round (ish) and has about 200 HP. Its saves aren't great, but aren't complete garbage. It has decent senses, and a burrow speed.
A naive level 20 rogue does about 40-50 damage per round with higher accuracy than the Remorhaz and has 122 HP. The rogue has a few defensive abilities, including uncanny dodge, that make it tougher than it looks. Its save are going to be better. It has quite impressive skills.
The AC of the two (without magic items) is going to be similar.
With (typical) magic items on the rogue and a subclass, I could see it going either way.
There is enough balanced fudge-room in D&D (and 5e in particular) that being more accurate than a certain amount produces false confidence.
Like, we might be better off if monsters where tier-based instead of CR or level based.
T0 monsters are level 1 and under, T1 are level 2-4, T2 are 5-10, T3 are 11-20, and T4 are 21+. Each of these windows is about a factor of two in power. If we stick the monster in the middle of the window (so 1.4x bottom and 0.7x top), then add in Minion and Elite(X), we'd have a toolkit.
Ie, tier based monsters look like:
L0.7 equivalent (roughly)
L3 (roughly)
L7 (roughly)
L14 (roughly)
L28 (roughly)
Elite(X) scales HP by a factor of X and damage by a factor of (X+1)/2, adds +2 to d20 rolls, and gives Elite Actions.
Instead of the "type modifiactions" of the linked system above, I'd have packages you add on to monsters that are strict bonuses. Then adding a package makes the monster tougher.
A side effect of tier based monsters is that the DM
cannot trivially use a sliding scale of monster power to nullify PC advances. Instead, each tier is narratively distinct. It resists the temptation to have Orcs just auto-gain levels as your character gets more powerful. Instead, you fight more Orcs, or something narratively distinct (a full tier up) like Ogres.