I think the difference here is that MIL didn't come out of her room when expected. After about 10 minutes, I think the OP would be okay to knock. MIL said OP didn't need to knock to wake her up and OP didn't. However, when MIL is significantly late exiting the room to get things going then a kind knock can let her know that not only are they awake but they are also ready for things to get going.
Perhaps I think that MIL realized baby was awake but didn't realize that everything was ready for the morning. She may've thought that the morning routine (changing the diaper, maybe a breakfast bottle, etc) took longer than it did and she thought she had more time between the baby's wake up cry and ready for presents. It sounds like she was ready in the room but didn't realize that the family was ready for her.