For one it's not hard to wrap your head around and for another thing defining Immortality as a "Shield to protect your mortality" is quite frankly asinine.
When it comes to things in relation to concepts meaning tends to hold more sway and by definition Immortality and Mortality are direct opposite in meaning. Immortality isn't a "Sheild" to protect mortality by definition it is a lack of the aformentioned mortality but due to our relatively broad categorization of what is considered "Immortality" and what qualities are inherent to mortality it varies a lot in fiction.
You're over complication what's very basic - Simply put there are 2 qualities "Immortality" and "Mortality" possesing one of these qualities in some sense inherently means lacking the other in some complementary way due to their direct opposite meaning.
There's some wiggle room when it comes to Immortality and Mortality because what we qualify/consider as immortality actually varies a lot ranging from simple having an inexhaustible lifespan while still being able to die through external conditions to other more complex situations like being able to reincarnate/ressurect, body snatching after critical damage or ... lacking the concept of death etc.
What Tiamate has is a lack of the concept of death which on it's own simply serves to immunize her from methods of instant death supernaturally brought about on her that do not forcibly impose that concept on her first while supplementing her regenerative type immortality.
Her lacking the concept of death still falls under the categorization of "Immortality" because immortality just inherently means lacking mortality - The degree to which a being posseses immortality is subject to how their immortality functions so naturally lacking death would be subcategorized as a form of "immortality".
Lacking the concept of death doesn't mean she's beyond immortality - The very notion of lacking the concept of death is just inherently a form of immortality.
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