Where does it say that?
It sounds like a very odd interpretation of "The GM has the creatures’ statistics."
I mean technically if you really only got to choose " One beast of challenge rating 2 or lower" as the option and not the actual beast summoned, the GM could summon one CR 0 Frog and still meet your request.
You pick the actual beast you want, and the GM just has those statistics because they have the MM.
The Conjure * spells state that the thing the caster is picking is one of the options, like X Beasts of CR Y. Intent is there, but it can be misleading, especially to players of other editions where you could pick the result. So they clarified in the official Sage Advice document on the Wizard's site. This is the one as official as errata, not the twitter.
When you cast a spell like conjure woodland beings, does the spellcaster or the DM choose the creatures that are conjured?
A number of spells in the game let you summon creatures. Conjure animals, conjure celestial, conjure minor elementals, and conjure woodland beings are just a few examples.
Some spells of this sort specify that the spellcaster chooses the creature conjured. For example, find familiar gives the caster a list of animals to choose from.
Other spells of this sort let the spellcaster choose from among several broad options. For example, conjure minor elementals offers four options. Here are the first two:
• One elemental of challenge rating 2 or lower
• Two elementals of challenge rating 1 or lower
The design intent for options like these is that the spellcaster chooses one of them, and then the DM decides what creatures appear that fit the chosen option. For example, if you pick the second option, the DM chooses the two elementals that have a challenge rating of 1 or lower.
A spellcaster can certainly express a preference for what creatures shows up, but it’s up to the DM to determine if they do. The DM will often choose creatures that are appropriate for the campaign and that will be fun to introduce in a scene.
