Clarification of Rule 17.4

[Graham Andrews]

According to the description of the rule 17.4:

Array indexing shall only be applied to objects defined as an array type.

.
Given this I do not understand why the following is allowed:

Code:

uint8_t a[10];
uint8_t *p;

p = a;
p[5] = 0; /* why is this compliant? */