That replies upon the definition of “mountain”, amongst other things, which has a consensus but isn’t something that is absolute. I agree that there is an objective nature to reality (though that can be argued too but I don’t see the value of doing so). I also agree with the opinion that Mt. Erebus exists because we are sharing an understanding of what is around us and agree on the evidence that comes from objective reality. However, you stating it makes it an opinion. It is an opinion based on evidence.

Personally, I think that there is an objective reality, but everything has to pass through our senses and understanding, so anything that filters out the other side is opinion. I don’t think facts are that absolute, but it’s a convenient way to say “this opinion has so much evidence to support it that I am confident it is true”, so calling things facts has value.

People (unfortunately) believe that things are facts without them being true, or due to them being a decent approximation, all the time. The thing that matters is the weight of evidence behind the opinion, as not all opinions are equal. I think that dismissing something as an opinion is silly, but dismissing one for lack of evidence or the fact it is subjective is valid.