Not true (as Hussar elaborated), and monsters don't have levels in pre-4th Ed, and CR is not accurate.
Okay, now that's
a weird statement.
P.10 of Dungeons & Dragons III: The Underworld & Wilderness Adventures has the "Monster Determination and Level of Monster Matrix", which cross checks "Level Beneath the Surface" and six "Monster Level Tables".
P. B4 of Moldvay Basic: "A 'monster level' indicates how tough and ferocious a type of monster is. A monster's level is equal to the number of hit dice
(a measure of how much damage a monster can take and still survive; see MONSTERS
, page B29) it has. Some monsters have special powers and the DM may consider them one "monster level" or (or hit die) higher than the number of their hit dice.
P. B29: "'Hit dice' also gives the level of the monster and the dungeon level on which it is most commonly found. In general, a monster's level equals its number of hit dice, ignoring any pluses or minuses. EXAMPLE: A monster with 3+1 hit dice is a third level monster and is most commonly found on the 3rd level of any dungeon. Note: if a monster has several special powers, the DM may consider it one level greater than its hit dice
"A monster's level is only a guide, and a monster could be found anywhere in the dungeon, whatever the level. However, as a general rule, it is useful to limit monsters to 2 dungeon levels higher or lower than their hit dice. When monsters are encountered on dungeon levels less than the monsters' level, there should be fewer monsters than normal. And when monsters are met on dungeon levels greater than the monsters' level, there should be more monsters than normal."
P. 63 of Mentzer Basic Rules Player's Manual: "monster level --
A measure of how tough a monster is, usually equal to its hit dice."
P. 22 of Mentzer Basic Dungeon Master's Rulebook:
"A monster with 1 Hit Die is called a "first level" monster. A monster with 2 Hit Dice is a "second level monster", and so forth. Any "plusses" are ignored.
"Monsters are encountered more often on the dungeon level equal to their level.
"Therefore, most of the Goblins encountered by a party will be found on the first level of the dungeon. Goblins will be encountered less frequently on other levels of a dungeon.
"If encountered elsewhere in a dungeon, the difference between the monster's level and the dungeon level is usually no more too."
p. 174 - 170 of the AD&D DMG is made up of "Monster Level" tables for encounters.
p. 98 of the 2e DMG states:
"Dungeon encounters are normally set up according to levels -- 1st, 2nd, 3rd, etc. Each level is a relative measure of the power of those creature on it. In general, the level of the table corresponds to character level, although characters may also encounter and defeat (or be challenged by) creatures from higher or lower level tables. Generally, when adventuring in a dungeon, characters should meet random encounters that are equal to or no more than two levels higher or lower than their own.
"Sometimes dungeons themselves are arranged in levels (although this is by no means required). In this case, the dungeon level and the encounter table correspond. Characters on the 1st-level of the dungeon would encounter creatures from the first level encounter table. This not only keeps the power of the monsters in line with the strength of the typical party, it also maintains the logical structure of the dungeon level. It doesn't make much sense for extremely powerful monsters to mingle freely (and without consequence) among the weaker creatures that inhabit that level."
Of course, none of this takes away the point that 4e's 1/2 level scaling tends to narrow the range of usable monsters (without adjusting their default level). But every edition since the beginning has given monsters levels to help DM's create encounters that were a decent match for the party.