[keane]

The goal of Zen and all Buddhism is to achieve "Enlightenment", a state of mind where one fully internalises the rather Daoist idea (remember the Yin Yang) that God does not exist and due to this Good and Evil are not different at all (an illusion) but are one and the same. Believing that Right and Wrong are equivalent is the same as denying they exist at all, hence Nihilism.

All of the practices done before this (meditation, compassion, tantra, etc) are--as in other branches of Hindu and Dharmic faiths--understood to be merely optional "yogas" or activities to fill the time and enjoy oneself in a Nihilistic world.

[locopati]

This is a complete misunderstanding and misrepresentation of Buddhism, which is the Middle Path between nihilism (a denial of existence) and eternalism (a belief in permanence or independence). Among the foundational beliefs are impermanence, interdependence, and the emptiness of self-existence all phenomena; or from another angle, suffering, the cause of suffering, that suffering can cease, the means of achieving that cessation. The practices are not meant to "fill the time" but are the means of seeing reality clearly.

[keane]

Not really. First off, Nihilism does not deny that we or the universe exist but instead "Nihilism is the belief that all values are baseless". In Buddhism, both good and evil has no real existence, being part of the illusory world of phenomena. As I said before, believing that Right and Wrong are equivalent is the same as denying they exist at all and this is what I meant by Nihilism.

Additionally, the practices of Buddhism are said to lead to Enlightenment, but even for the Buddhist, wanting Enlightenment is a huge mistake as Moksha/Nirvana is available at all times, without the practices, as our default status. As Jed McKenna notes: Why does Buddhism rarely produce Buddhas? It is because Buddhist practices ("yogas") are not able to transmit the realization that Self is Illusion and All is Nothingness. If you read the Anathapindika you'll see that Siddhartha denied that the Creator God (issara-karaṇa-vāda) existed, that he claimed there was no free will, but even though every thing is deterministic we should practice good anyway.

  Let us, then, abandon the heresy of worshipping the Creator God and of praying to him;
  let us no longer lose ourselves in vain speculations of profitless subtleties;
  let us surrender self and all selfishness,
  and as all things are fixed by causation,
  let us practise good so that good may result from our actions.

Rather than attempt to understand Buddhism as if it exists in a vacuum, studying the wider Hindu philosophy that it developed from and especially Nondualism and Vedanta will likely help you understand what Siddhartha was trying to convey. It is absolutely Nihilistic.