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It's true there is a clause stating extradition from Hong Kong to China cannot be based on political motives. However, there are worries the Chinese government would fabricate charges just to get dissidents over to China, as they have done or tried to do before. In general, it's very problematic to determine whether the charges are truly not political.

There is also no guarantee of a fair and democratic judiciary process in China.

To read more, I recommend reading this article:

A proposed extradition law triggers unrest in Hong Kong

Hong Kong-China extradition plans explained

What is Hong Kong’s extradition bill?

The key is that extradition hearings are not trials that establish innocence or guilt. From your quote, the requirement for extradition is that there is:

sufficient prima facie evidence that there is a possible case

To people who think that China is willing to occasionally forge evidence and hold mock trials, that is equivalent to "prolonged imprisonment or death, as long as China makes an effort to come up with an excuse".

It would be very difficult for China to extradite anyone as Hong Kong does have political autonomy and an autonomous legal system. Even if the Chinese government did forge documents, the extradition request can be denied on other grounds and the process could take years, and it's unlikely for the Chinese government to pursue an extradition case against an ordinary citizen for political reasons.

If the justice department determines that the conditions are met, then it goes before Hong Kong’s chief executive, who can decide whether to veto or proceed with the extradition request. At this point, the suspect can apply for judicial review, with a right to appeal in the city’s highest court. If the request proceeds, an arrest warrant is issued, after which the subject is immediately barred from leaving Hong Kong. Once the subject is arrested, the case moves to the courts, where a preliminary hearing is held. Once the court decides that there is no political motive behind the extradition request, and that there is sufficient prima facie evidence that there is a possible case, it can then make an order of committal. At this point, the suspect can appeal.

With the judicial process over, the request goes back to the chief executive, who can again decide to deny the extradition request on humanitarian grounds. Here, the subject can petition the chief executive to oppose extradition. Should the chief executive decide to proceed with the extradition, an extradition order is given. At this stage, the suspect can again appeal to the courts to stop the extradition. If the suspect decides not to appeal, or if the appeal is unsuccessful, the suspect is extradited. The process could take years, which is typical of extradition proceedings.

https://qz.com/1635504/everything-you-need-to-know-about-hong-kongs-extradition-law/

As you can see, there are many checks that would make extradition extremely difficult. Here's a summary:

1. The chief executive decides to proceed with the extradition request
2. The suspect applies for judicial review
3. The suspect appeals in the city's highest court
4. The court decides that there is no political motive behind the extradition request
5. The suspect appeals in the city's highest court
6. The chief executive decides if there's a humanitarian ground to deny the extradition request
7. The suspect appeals to the courts to stop the extradition