The Wikipedia page about Shor's algorithm currently contains the following sentence:
A complete factoring algorithm is possible using extra classical methods if $N$ is a semiprime, that is, if it is the product of just two primes $p$ and $q$; therefore the algorithm only needs to achieve that.
The sentence is not very clear, but if I'm interpreting correctly, they are saying that if we are able to factor efficiently semiprimes, then we are able to factor efficiently any integer, using "extra classical methods". I was however unable to find a reference or other mentions of this fact.
I found a few places discussing how/why factoring semiprimes is particularly difficult, for example this post on crypto.SE, and other relevant Wikipedia pages such as Integer factorization and Semiprime. I didn't, however, find mentions or references to the statement that the Wikipedia page seems to be hinting at. Is the statement accurate? If so, why techniques are used to obtain this reduction?