Sentient magical items are objects and thus cannot take actions on their own and thus are considered an ally the same way your tunic, bow, boots or belt are considered an ally. Which is that they are not.
Thank you @Rubiksmoose for this Tweet as well. The line of reasoning points to only creatures can be allies as it states that spell effects are also exempt. Certain spells call out that they actually create temporarily or otherwise a creature, like Simulacrum, Tiny Servant and Animate Objects.
Further evidence to support this is that Pack Tactics indicates that the ally cannot be incapacitated and the conditions in the PHB 291 indicate that creatures (not objects) have conditions.
I realize this evidence is a tad soft but it is what it is, and personally some of them should affect a sentient item such as Blindness. This has no bearing on combat obviously but definitely on its senses and ability to locate things based on sight. So you could (like I will) rationalize that it applies the equivalent to that condition if you want to get really, really technical.
All of the spells that I know of that apply a condition also only target creatures.