and they are explaining to you that once you apply for a non-immigrant visa, you will be abandoning your permanent residency.
If you need to stay outside of US for more than a year and less than 2, you have to apply for reentry permit. If you intent to stay outside the US for more than 2 years, you will abandon your US residency, and have to try to get a non-immigrant tourist visa. Whether you will succeed is unknown.
If you come back to the States at least once a year and you will have proof that you continue residing in the US and are abroad for temporary visits, then your GC sort of works as a visa.