Is "bitcoin" a countable noun? Should "bitcoin" be used as a countable noun admitting a singular and a plural (so that one says "600 bitcoins are in this wallet") or as a mass noun, (like "water" or "money") (so that one says "How much bitcoin is used every day throughout the world" or "600 bitcoin is enough for this transaction")?
Everyone is using it as a countable noun. There's actually a grammatical rule that says this is what should happen. If a new term ends with an existing term and is semantically related to that term, it gets the old term's rules. If not, it does not.

So, a "pop fly" is not a type of fly (the bug). So we don't say "pop flies" but "pop flys".

A "bag lady" is a type of lady. So we don't say "bag ladys" but "bag ladies".

We don't say "two firemans" because while a fireman is not necessarily a man, the origin is directly related.

So, a "bitcoin" is a type of coin, the terms are semantically related. So it should inherit the rules of the word 'coin'. Since we'd say "one coin", "two coins", we would expect the rule to be "one bitcoin", "two bitcoins". This feels natural to people, so it's what they do.