Nowhere in the description to the petrified condition does it say that a creature becomes an object
In 5e, effects do only what they say they do and anything beyond that is up to the DM. There is nothing in the petrified condition that would indicate that a creature would get turned into an object. Objects and creatures are very important classifications and are always called out when targeting or changing from one to the other. Since the condition does not say anything about changing a creature to an object, we can thus conclude that that is not an effect that is included RAW.
For an example of an effect which does actually turn a creature into an object we can look at true polymorph:
The creature's statistics become those of the object, and the creature has no memory of time spent in this form, after the spell ends and it returns to its normal form.
Without this explicit language, the RAW does not support a petrified creature being an object.
A petrified creature is still a creature
All the evidence that is needed to deduce this comes from the language of the petrified condition itself.
The initial sentence describes the overall effect of the condition:
A petrified creature is transformed, along with any nonmagical object it is wearing or carrying, into a solid inanimate substance (usually stone).
After that, all other sentences refer to the affected creature as "the creature". This means that the condition itself is explicitly saying that the petrified creature is still a creature. A notable mention from the condition is:
The creature is incapacitated (see the condition)...
Note that objects cannot be incapacitated (or any other condition) giving further support to the argument that they are not, in fact, an object.
Jeremy Crawford unofficially agrees with this in this tweet:
Neither the petrified condition nor the flesh to stone spell turns you into an object. You are a creature subjected to the petrified condition (PH, 291).
No condition, including the petrified condition, changes your creature type.
Thus, becoming petrified does not turn you into an object.
Siege Monster will not do double damage to a petrified creature
The Siege Monster ability says:
does double damage to objects and structures
Since petrified creatures are not objects, they do not take double damage.