I call that the milk-march. Some cats do that way up into their old age. They seem to do it when they are near someone they “love”, and they want a good pet session. Mine also salivate heavily once the petting starts. A little of that goes a long way; but it sure beats their reputed indifference
I call it “making biscuits,” and it’s definitely a sign of affection. Misty and Deimos are middle-aged and both still do it, the former more so than the latter.
My Stella salivates when she’s kneading me, too! She gets this drip going on her chin and it’s just so cute. It definitely is a sign of affection and I count myself privileged to have it done to me.