Is the loss guaranteed if we hold it for a long period of time??
I still don't understand why
Thats why:
It is clear that the risk & reward ratio between them is very high.
If we bet btcup, we only can win 20times, but if we bet btcdown we will lose 300times.
When trend will reverse and BTC will start to dump couple % daily being BTCdown will not gives you 300x. It will give you 30x while Btcup will be under x300 risk.
It is explained in the link you already provided:
https://www.binance.com/en/blog/421499824684901079/Why-you-shouldnt-hold-leveraged-tokens-longterm-"Leveraged tokens are built to multiply the underlying asset's daily return-the main component to remember here is DAILY. The leverage factor of a token will be reset every day. As a result, the performance of a token and its underlying asset can differ over the long term."
If BTC will dump 30% you will lose 90% (100$ left from 1000$ investment). Next day BTC will do +50% (back where it was), you portfolio will do +150% (250$). BTC recovered the dump but you are still 75% under water.