Watching Death Note with my partner and they asked "How do you know which Death Note is yours?" And the obvious answer seemed potentially fatal.
My interpretation is that "the" and "a" is doing a lot of work there. Maybe a translation quirk? It reads like, if it's misspelt 4 times in THE death note (the one you're holding), but later on it clarifies that it doesn't make you immune from ANY death note. So it's not so much complete immortality as limited exclusion to prevent abuse, which makes sense if you imagine some Eldritch death God being spammed with junk assassination requests. Your specific death note got a filter applied because you were being dumb