Surely by literally saying "formerly Twitter", this is an admission of non-use, thus invalidating the trademark on the term as it is not just "not used for anything" but is explicitly being distanced from by X in writing.
Ex. Google Wallet -> Android Pay -> Google Pay -> Google Wallet [1]
A big difference here is that X has been pretty clear on it has no intent to launch a product called Twitter. I doubt you can find similar statements from Google saying they'd never launch a Google Wallet after the Android Pay rebrand.
And Twitter being renamed to X is literally that: the company and product name were changed, and both the company and product no longer uses that original name for anything other than to call out "formerly Twitter". That is not "use" for the purposes of keeping your trademark.