Earlier quoted context omitted.
One person reaching 2900 when it's not by changing the scale shows their relative position to others is a new record. it's a worthy psychological goal, like breaking a 4 minute mile even though a minute and a mile are made up quantities. Or scoring 300 in bowling, when 10 rounds is arbitrary. Pretty much every record in every sport is arbitrary, yet people like to set goals for motivation.
Yes but the examples you mentioned are (in practice) absolute and can be reached independently of how others are performing. In that sense, ELO _is_ different (as also explained in another answer, he can reach 2900 more easily if others start performing worse).
Federer getting grand slams - all against local in time competitors - is impressive. Had he played 50 years ago, he'd have likely gotten more. Had he played in 50 years, likely less, expecting competition to improve.
Same for NBA records - all against a moving, relative backdrop.
Same for wrestling, baseball, hockey, and on and on and on.
Just because your score is relative to others does not make reaching some milestone, especially this one, less impressive.
Also, it's easier to have a higher ELO if others do too, not the other way around.