What does the acronym ‘PIN’ stand for referring to PIN proteins in plants?
Like many genes and gene products, PIN proteins were named for a mutant phenotype and PIN is not actually an acronym; the source in your link does actually explain this (emphasis mine):


  The significance and function of AtPIN1 was discovered through the phenotype generated by the loss-of-function mutation in the gene: mutant plants fail to develop floral organs properly and generate naked, pin-like inflorescences, which gave the name PIN-FORMED (PIN) to the family


Křeček, P., Skůpa, P., Libus, J., Naramoto, S., Tejos, R., Friml, J., &amp; Zažímalová, E. (2009). The PIN-FORMED (PIN) protein family of auxin transporters. Genome biology, 10(12), 249.