multiverse is a seprate concept from timelines.
For example, did you watch the first dr. strange? In that film , there was a completly disconnected space from earth's reality which was lead by a godlike figure named Dormamu. That complete diconnection between realities is what defines a universe , within the mcu multiverse.
Theoretically , each universe would have its own timeline, but not necessarily sacred timelines; cause there are universes without kang, such as dormamu's world where he is the only living entity, so kang cant keep a specific timeline there caus ehe doesnt exist. Ironically, that universe form Dr.Strange1 is a universe that doesnt even have the concept of time, hence why strange was so powerful there with his time infinity stone.
If i had to describe the timeline thing, i would compare it to a starfish. Stay with me here, i know its a weird anology, but it will make sense.
Did you know that many starfish can cut themselves in half and then grow two of themselves from those shattered halves? In this instance, both halves of the original starfish are effectively the exact same starfish from a gentic standpoint, but they are seperate entitities if they were stacked next to eachother. Think of the divided starfish as a split timeline in LOki.
Conversely , A universe would be its own seperate species. Rather than having both of those divided starfish sitting next to eachother; a universe would be like putting a a seahorse or crab next to starfish.