Wikipedia has a very nice article on what the term meant when "saeculum" was adopted into Latin from Etruscan, and Studies in Words has an extensive section on mundus/saeculum/ecclesia which explains in even more detail how "saeculum" can also be understood to mean (a time-bound) "world" in certain (primarily biblical) contexts.
As I'm not a fluent speaker of Latin, these are the primary resources shaping my belief that "saeculum" has some rather different connotations than the English word "generation" which is otherwise closest to it from our modern translation options. The difference is a defining event which starts a saeculum. For example, the people on board of Mayflower who landed in Massachusetts in 1620 formed the first "saeculum" (generation) of those colonists. In standard English usage, the second generation colonists would be their children born in America. Whereas the second "saeculum" of the colony started only when the last first generation colonist died, and was formed by exactly those colonists who were alive at that time; and it lasted exactly while any of those people were still alive. Enter third saeculum. And so on.
So, a "saeculum" lasts much longer than a "generation" (of people) in common English usage, and it has a definite starting point in time, a point, if I may try to suggest a metaphor, when it was "sown" from the immediately preceding "saeculum".
Latin and Etruscan are Italic languages. If PIE *seh₁- ("sow") provided the first part of whatever the proto-Italic word was, the second part is more widely recognized (another reference - see page 12) to be *-tlom (instrumental suffix). The evolution of *-tlom into Latin -clum or -culum is accepted rather widely and your reference assumes it, too.
Note that the last referenced book, on pages 14 and 18, also suggests some reflections for this combination of the two reconstructed morphemes (i.e., an instrument for sowing) to be either a sieve or the seed being sown. I can't quite see any semantic evolution of a sieve into "saeculum", but the latter meaning, seed, doesn't look too disconnected from the metaphor suggested above.
Note that I'm purely speculating. I don't know who may have suggested *se- or *seh₁- as the origin of the first part of "saeculum" and what their thinking was. I just like to entertain the possibility that the Etruscan/Latin meaning could have been somehow agriculturally inspired: crops are grown in disjoint "generations", unlike people.