I think defining Hinduism is a really complex affair. Is nastika really not Hindu, or is it just an unorthodox philosophy? The ancients seemed unconcerned with this, and in their texts they treated nastika and astika philosophies fairly similar, including atheism. The ancient Indians were far more concerned about the nature of God and the universe than on these definitions, and they viewed philosophies within nastika and astika, such as Advaita Vedanta (the dominant philosophy of Hinduism today), much more importantly than astika vs nastika.
Actually the ancients really did care about the difference.
First of all to clarify, the way it describes Orthodoxy (as the wikipedia article discusses it) is misleading. It isn't so much orthodoxy in the sense of "this is the correct way to think" (notably Buddhism formed it's own internal orthodox heterodox system) but orthodoxy in the same way that being a Christian in the UK is fairly normal whilst being a Taoist is quite unorthodox. This is important a distinction to recognise as it is why we can consider something like Samkhya as Astika because it conforms to the orthodoxy of the Vedas even if it's practitioners were something of a minority within that orthodoxy, as opposed to Jains and Buddhists who were distinctly seperated despite being more populous than Samkhya followers.
The idea that the ancients were not concerned with this, or that the other native indian religions fit somewhere into Hinduism is a modern conception which is used by Hindu nationalists much to the annoyance of Buddhists, Jains and Sihks in India, it being easier to marginalise their rights as distinct religions if they are not recognised as distinct religions.
In reality, important figures in Jainism and Buddhism (and later Sihkism) not only were defined as others by their rivals, but defined themselves so as well. Nagarjuna in particular is a fun example to study in this regard, being something of a renegade for his time within the Buddhist tradition (being the founder of Mahayana) but recognised as a Buddhist by his contemporaries and in regular debates with his Hindu and Jain rivals. Whilst they were contemporary and there is a lot of overlap (Hinduism in particular having the ability to absorb Buddhist and Jain ideas and vice versa), they are distinct in the same way we can recognise Cynicism, Stoicism and Epicureanism as distinct philosophical schools of ancient greece as opposed to different sects of the same philosophy.
Later, there are even more matters on what can be defined as Hinduism. Was Sikhism in the time of Guru Nanak a form of Bhakti Hinduism? He is a Bhakti saint, after all. And when did Sikhism truly become its own religion? In the time of Guru Gobind Singh?
Sikhism is more of a challenge, but I would say it was certainly distinct by Guru Gobind Singh.
It is really difficult to decouple nastika from Hinduism.
Not really. It is self explanatory in the game =L