I think it's been a reasonably significant problem in the past for members to inform other members that the type they identify with is not their type. I don't think that people who respond to others this way necessarily do have the intention of providing an alternative perspective. Actually, I have no idea what their intentions are, except that they clearly have the intention of informing others of the type that they have personally concluded that those others are.
I think it is poor etiquette to give others unsolicited feedback on their personality. Would you confront an acquaintance with remarks to the effect of something like (for example), "After hearing what you just said, I think that your personality is directed more towards sensory reception than symbolic meaning"? Maybe you would do that... but if you would do it, I think that it would generally be considered to be impolite.
What I have usually understood when engaging in typology is that typing is invalid without the personal identification of the person who is typed. Often when I have seen people typing others it is something like "You aren't what you think you are! You are some other type entirely!" The people making remarks like this seem to have lost perspective on this idea that typing is only valid if it is provided by the identification of the person who is being typed.
Once before I saw that on another typology forum, the practice of providing unsolicited typing feedback to others had caused so many problems for the community that it had been given a special name and had been explicitly prohibited. The name that it was given was called something like "type flaming" or something, I can't remember exactly. I'm sorry that my memory is so poor about this, but I relate this anecdote because I think it is meaningful. I think it is something that is worth bearing in mind.
There are people who post threads asking, "What type am I?" and that gives others opportunity to provide feedback on these matters. Outside of that? No, I don't think it's very polite to spontaneously provide unsolicited type feedback to others. I think that it is polite to respond to the content of what people post, and to refrain from making inferences about the personality of the person who contributed to discussion from what they posted.