As a data analyst, I can tell you that data can be ordered any number of ways, but whatever "order" you choose it is always "sequential" which literally means that the order follows a sequence.
- Blue, Green, Red (alphabetical order)
- Red, Blue, Green (descending order according to their Terahertz value on the colour spectrum)
- Blue, Red, Green (by popularity of car colours)
I don't believe there can be such a thing as "non-sequential" ordering, because if any items were listed without a sequence then they would not be any order.
If items were ordered entirely randomly and you were to "index" them as you found them, rather than rearrange them into some kind of order, then it would simply be "an index", so that you could locate them. Arguably though, by creating an index of randomly placed items, you have by doing so created an order. For this reason, I would argue that there is no such thing as "non-sequential indexing", and even that "sequential indexing" is something of a redundancy.
As your quote is defining the concept of "time", it might be worth noting that the term "linear time" is used to describe the nature of time, and while the term "nonlinear" is used, it is considered to be an abstract concept - nothing more than the way people perceive linear time differently.