She hadn't become a mutant yet, and since Terrigenisis can happen at any age, she became an inhuman first, and they claim it suppressed her X-gene. The X-Men knew it all along and were keeping it from her until she was resurrected because they ignored the Avengers vs. X-Men vs. Eternals event, and claim that only mutants could be resurrected, and therefore were able to resurrect her and had to explain why she was resurrected, revealing to her she was a mutant-inhuman hybrid.
The thing is that this is telling us that there are a lot of inhumans that were hybrids and were supposed to have their X-gene activate, but their inhuman DNA manifesting during global Terrigenisis blocked their X-Gene from activating during puberty.
Technically, there are a lot of inhuman-mutant hybrids, they just need a mentor, which is what Professor X said he was going to be to Kamala, to help them manifest their X-Gene on top of their already manifested inhuman DNA.
They can use this as a story, or as a cheap gimmick to explain this marketing ploy.