You asked in your original query whether bbc was a reliable source; it is. Well to an extent. It is instantly recognisable but it is fairly biased. In any case a biased narrative can be just as misleading as false news so take everything you read from the bbc with a huge grain of salt.

As far as the article you have linked goes, it makes some good points. The drug business is one thing, the taliban also have extensive human trafficking and exort protection money "taxes" on people and businesses. They also smuggle goods across borders and take percentages. Kidnapping and ransom money (especially of foreign nationals) was very common in the earlier days for making huge sums of money at a time. There is also the fact that the Taliban actually have a lot "legitimate" businesses or just funding from powerful people across the world.

All the relief and development spending from countries like India in Afghanistan also inadvertently add to the Taliban's pockets. Most contractors of infrastructure pay "protection money" to the taliban even with all effort of the Afghan government to stop the practice.

What bothers me the most isn't the source of funding. You can find a thousand sources if you are smart enough. No the actual question is how are the inexperienced and uneducated leaders of the Taliban (at least the original ones) capable of even handling, moving around and just generally looking after such a vast network of finances.

By all account everyone admits that the Taliban have a substantial stream of income. It might not be a staggering amount but it is still impressive given the circumstances. Those revenue streams are also sustainabile to such an extent that most everyone has given up trying to get rid of the Taliban and instead moved to making some sort of agreement with them.

The taliban are here to stay is the clear message but the financial side of things is very very interesting. I genuinely would love an interview with the mastermind behind all of this in a few years (some western news outlets even claim that the ISI is behind it all. Good lord like we dont have enough of our problems to deal with). The Taliban have held up financially for the most part of two decades! I'm not their biggest fan but this is something worth looking into.