No. Those are the assumptions made by people calculating DPR for this stuff. There's zero indication that WotC does this. They might, but they haven't said either way. It's just common practice.
Well, Teos said he actual asked people at WotC if this is how they still do it in 2024) and they said yes. However, the 2014 DMG strongly implies it:
"
Overall Damage Output. To determine a monster’s overall damage output,
take the average damage it deals with each of its attacks in a round and add them together. If a monster has different attack options, use the monster’s most effective attacks to determine its damage output. For example, a
fire giant can make two greatsword attacks or one rock attack in a round. The greatsword attacks deal more damage, so that attack routine determines the fire giant’s damage output.
If a monster’s damage output varies from round to round, calculate its damage output each round for the first three rounds of combat, and take the average. For example, a
young white dragon has a multiattack routine (one bite attack and two claw attacks) that deals an average of 37 damage each round, as well as a
breath weapon that deals 45 damage, or 90 if it hits two targets (and it probably will). In the first three rounds of combat, the dragon will probably get to use its breath weapon once and its multiattack routine twice, so its average damage output for the first three rounds would be (90 + 37 + 37) ÷ 3, or 54 damage (rounded down).
"
Also, if you follow these assumptions, with the rest of the guidelines, the CR calculator works. So, it is pretty clear actually.