Musk is claiming that it recently reached record usage highs, as measured by "user seconds". It's a bit unclear whether he is counting X as a different platform to Twitter though, and "user seconds" is pushed upwards by video content that takes longer to consume. But usage hasn't obviously declined since the takeover either.
Hint: when people start using a weird metric that no-one has ever hard of before, they are generally doing so because real metrics are, ah, less favourable.
For the ultimate example, see WeWork’s old ‘community adjusted EBITDA’, which was pretty much just revenue.
I don't think it's that weird assuming it's defined in the obvious way. Such a metric is obviously harder to game than MAU or similar as it's less affected by occasional users who don't really use the site but click through links from time to time. It also seems more aligned with what advertisers care about (how many seconds are spent looking at ads).
https://x.com/elonmusk/status/1854777813464662307