To try and explain a bit, at the end of the original series Xavier was mortally wounded by Gyrich and taken to space for healing by Liandra. Access to Shiar medical tech would likely be needed to keep him alive, so he was expected to be gone long term. Both the XMen and Magneto were aware he left, but after over a year with no communication they may assume he's dead.
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I find it kinda bussin, I find it kinda cap That the dreams in which I'm dying are the ones that kinda slap