It isn't referring to having friends, that's a misunderstanding of the terminology. It is referring to a person building a dynamic with one specific "friend" that must be done with secrecy to prevent damaging or ending the original relationship, and the person putting time, effort and energy into that secund relationship to the detriment of the first, there secund usually had all the hallmarks of an early stage relationship, only it has not yet moved to the nevermind of physical/sexual. Sharing emotions etc with a platonic friend you may have certain private conversations with them but you don't need to hide every interaction you have with them out our shut your vibe with them to your s/o. People aren't stupid, they know very well if they're talking to someone as a genuine friend or if they're developing a dynamic which would deceive and hurt their partner.