One of the most commonly misunderstood, misinterpreted, or just plain missed things in the canon is the question of what, exactly, is going on with Chisei.
Which is to say: why is his age in quotes, is he being serious when he said he died, and what is going on?
At some point, Chisei collapsed on stage because of phantometal erosion and Haruomi did a metal kiss mind-dive to save him. This is apparently not well documented; Iori talks about it as being somewhat apocryphal, and even Allen, Buraikan mega fanboy that he is, has never heard about it. (The fact that this is an urban legend, and that nobody seems to know why Buraikan disappeared, may indicate that this was not as successful as Iori claims.)
[source: VIBES Part 2]
CLUB Paradox's re-opening and the announcement of the Paradox Live starts the series, ten years after Buraikan's sudden disappearance. Buraikan have not been seen at all in the intervening ten years, so people are super skeptical of the idea that Buraikan are actually involved with this at all, even as we get to the end of Round 1.
[source: Opening Show tracks and the skepticism is again referenced in the Buraikan is Back feature]
Their first major in-person appearance, together, is in LIVE pt.4: Haruomi is revealed to be the old man who runs Raimentei, appearing much older than he should. Chisei, meanwhile, appears to have not aged a day in ten years when he appears.
Haruomi seems surprised to see Chisei, who says that it's "thanks to [cozmez] that I'm able to come out like this" and remarks that Haruomi has become an old ramen-shop owner "because I up and died and left you behind". (You'd think that this would be pretty incontrovertible proof, but it's a very throwaway line that nobody really dwells on, and it's hard to tell if Chisei is being facetious.)
[source: LIVE Part 4]
Though subtle, Chisei never has a reflection in any MVs where he appears, most notably in Rap Guerrila Reload and BURAIKAN is Back.
When Chisei appears in LIVE part 4, he is assumed to be an illusion. Haruomi, unlike Kanata, appears to be aware that he is manifesing an illusion of a dead person, but is keeping the illusion up for his own undisclosed reasons.
We know from phantom Nayuta that metal phantoms are capable of acting and speaking indepenently from the person generating them, and are corporeal enough to manipulate physical objects (see the LOVE dramas).
So, yes, when Chisei says "because I up and died", you should take it at face value.
Whether you take the anime as canon or not is your own cross to bear. Many parts of the anime, especially the parts from which we draw this information, are incompatible with the drama track canon, but if you choose to accept it, then it just straight up says that Chisei is dead, so thanks for that, I guess.
Visuals at the beginning of episode 11 seem to confirm that it was phantometal erosion that killed him. A line from the Alter Trigger scientist may seem to contradict this, thanks to the way the official subtitles say "the same way Yasha died" as he's firing a gun at Shura, but based on the actual line in Japanese, I believe this is probably meant to simply mean "you're going to die, too" and not a statement of how Chisei died.
The anime features a phantom Buraikan created by Alter Trigger for the purposes of the first Paradox Live; if we attempt to fold this information into the drama canon, we can assume that Haruomi continues to create the phantom Chisei on his own.