Why is this sentence correct?

Rules for Using Semicolons

1) A semicolon is most commonly used to link (in a single sentence) two independent clauses that are closely related in thought.When a semicolon is used to join two or more ideas (parts) in a sentence, those ideas are then given equal position or rank.

Some people write with a word processor; others write with a pen or pencil.

2) Use a semicolon between two independent clauses that are connected by conjunctive adverbs or transitional phrases.

However they choose to write, people are allowed to make their own decisions; as a result, many people swear by their writing methods.

3)Use a semicolon between items in a list or series if any of the items contain commas.

There are basically two ways to write: with a pen or pencil, which is inexpensive and easily accessible; or by computer and printer, which is more expensive but quick and neat.

4)Use a semicolon between independent clauses joined by a coordinating conjunction if the clauses are already punctuated with commas or if the clauses are lengthy.

Some people write with a word processor, tablet, or a even a phone; but others, for different reasons, choose to write with a pen or pencil.

Now coming to the sentence you quoted

Bamboo thrives in steamy areas; nevertheless its products require a lot of treatment to withstand sunshine.

This sentence falls under category two.

Use a semicolon between two independent clauses that are connected by conjunctive adverbs or transitional phrases.

nevertheless being the transitional phrase.

Hope this helps.