I don't know much about Pliny, but Pliny's treatment of Eratosthenes, which just precedes the section you quoted, indicates Pliny does understand the math. Pliny writes of it that:

it is supported by such subtle arguments that we cannot refuse our assent

Doesn't that indicate that Pliny had read and followed Eratosthenes' argument?

Pliny's later digression on a story about a letter seems irrelevant to this issue. The story doesn't look to be a substitution for a mathematical case; it looks like it's supposed to be only an anecdote.