Why not to use timestamp with Interleaved Sortkey?

I think it might have been explained later on when they describe issues around vacuuming/reindexing:

When tables are initially loaded, Amazon Redshift analyzes the distribution of the values in the sort key columns and uses that information for optimal interleaving of the sort key columns. As a table grows, the distribution of the values in the sort key columns can change, or skew, especially with date or timestamp columns. If the skew becomes too large, performance might be affected.

So if that is the only reason, then it just means you will have increased maintenance on index.

From https://docs.aws.amazon.com/redshift/latest/dg/t_Sorting_data.html

As you add rows to a sorted table that already contains data, the unsorted region grows, which has a significant effect on performance. The effect is greater when the table uses interleaved sorting, especially when the sort columns include data that increases monotonically, such as date or timestamp columns.

The key point in the original quote is not that that data is a date or timestamp, it's that it increases "monotonically", which in this context presumably means increasing sequentially such as an event timestamp or an Id number.