because he discussed this openly with others in a discussion forum.
Ah, that's interesting. Then his public statements likely contain a lot of relevant information to evaluating his claim. He's probably made a lot of other statements in that forum which would be important information to consider in working up a legal opinion.
You're saying it doesn't matters in a UDRP dispute that the domain name I purchased has existied since 2011 well before he TM the word. Fact is, I had no idea about his TM, his website, or his intent to eventually sell goods/services using the word.
The first part is correct. What matters in a UDRP dispute is your state of mind, as might be inferred by objective evidence, when you acquired the domain name. What someone else was doing since 2011 is generally not relevant.
While I certainly don't doubt that you acquired the name for reasons unrelated to whatever this guy is up to, you are the only one who knows what goes on in your head. The way the rest of the world determines intent is by looking at what you might have been aware of based on external evidence.
That, again, brings us to "what sort of a term are we dealing with," which would also be an important consideration in the event someone were seeking an actual legal opinion on a particular circumstance.
In other words, a UDRP panel is going to look at:
a. All the possible reasons why the respondent might have registered the name because it is the complainant's trademark, and
b. All the possible reasons why the respondent might have registered the domain name because of some other rationale,
...and decide which of those is more likely and supported by evidence. So if the domain name is a word, combination of words, surname, catchphrase, three- or four- letter string (and not something like IBM), there are reasons to believe that someone who registers names or is in the habit of registering names like that ("here's a list of similar other names I own") registered the name for reasons consistent with that rationale. If this guy simply had the word on a website somewhere and some kind of aspirational plan of getting around to using it as the name of some kind of goods or services at the time you registered the domain name then it is unlikely he would prevail.
However, if it is a relatively inherently distinctive term, then his junior (to your domain name) trademark registration some day in the future may diminish the resale value of your domain name to anyone else who might not want to buy a distinctive term that is already a registered mark.
That was the dynamic of the more than decade long standoff which ultimately worked out well for this domain registrant:
https://www.adrforum.com/domaindecisions/547889.htm