I think you are out of luck with their explanation. There are several words in the dictionary you can look up that use each other for definitions. Context is the key.
The below is from https://thecontentauthority.com/blog/span-vs-period
A span is a term used in various contexts to describe the duration or length of something. In general, a span refers to the amount of time or space that something occupies. For example, in construction, a span might refer to the distance between two supports in a bridge or other structure. In music, a span might refer to the range of notes that a particular instrument or voice can produce. In web development, a span might refer to a specific section of text or other content that is highlighted or formatted in a particular way.
The word “span” refers to the length of time or space between two points. When using “span” in a sentence, it is important to make sure it is being used in the correct context. Here are some examples:
The bridge spans the river.
The concert will span two hours.
Her career spanned over 30 years.
A period, on the other hand, is a more specific term that refers to a particular length of time. In general, a period refers to a duration of time that is defined by a specific set of criteria. For example, a period might refer to a month, a year, or a quarter. In science, a period might refer to a specific interval of time in a recurring cycle, such as the period of a pendulum or the period of a wave. In grammar, a period refers to the punctuation mark that is used to indicate the end of a sentence.
The word “period” has a few different meanings, but in the context of time, it refers to a specific length of time. When using “period” in a sentence, it is important to make sure it is being used in the correct context. Here are some examples:
The Renaissance period lasted from the 14th to the 17th century.
She was on bed rest for a period of two weeks.
The company reported a profit for the period ending June 30th.
In each of these examples, “period” is used to describe a specific length of time.