Toddlers unless they have a medical condition or bladder issue are not urinary incontinent.
As a toddler gets to toddler age they gain bladder and bowel muscle control.
Most toddlers are no longer urinary or bowel incontinent by the time they reach age 18 months of age although some toddlers may take a bit longer to develop good bladder and bowel control.
A 2 year old certainly can hold their pee and poop in and let the pee and poop out all at once.
My 2 year old can hold his pee and does so for a few hours until his bladder fills up and then he lets it out all at once.
Sometimes my 2 year old boy will be holding his pee in while in his diaper and then be squirming in his car seat or hold his diaper in front and then eventually releases the pee.
He's not ready to use the toilet yet but he certainly can hold his pee in.
I'm okay with him wearing diapers for as long as he needs too and he'll eventually get it.
However while toddlers are not incontinent newborn babies are incontinent when born and cannot hold their pee or poop in.
This is one reason babies need to wear diapers because they cannot control when they pee or poop and it comes out whenever and wherever.
If babies did not wear diapers then they would pee and poop all over the place.
As a baby grows and gets older the babies bladder and bowel muscles strengthen to where they can eventually hold the pee and poop in.
Most babies bladders and bowel muscles are developed enough to hold the pee and poop in when they turn 18 months of age.
Most babies were potty trained by 18 months back in the 80's and 90's.
I was using the potty by age 18 months because I could hold my bladder and poop.
When a baby is 18 months or older they use less diapers because they poop less and they pee less often.
That is because the baby is usually holding their pee in and then letting the pee out all at once at certain times.