While the California laws are in fact quite draconian and hard to learn, they are also relatively specific, or have been made specific by prior case law.
The result is that all the facts are there to determine what is and is not legal in most circumstances, unlike for example some of the federal laws dealing with firearms that are vague enough that you need an agency to interpret them at their discretion.
This means a cop can in fact determine what is and is not an assault weapon by feature if they know the law.
There was some guns banned specifically by name in the Roberti-Roos list, most of which is no longer important within the state because the companies have similar products now under unbanned names. (Unless you are moving into the state with a firearm from out of state that could be on the list.)
These weapons are always 'assault weapons' no matter what features you add or remove.
Another class is those specifically added by name before they stopped doing that some years ago. These are likewise always illegal in any configuration if they weren't registered before the cutoff, and can never be transferred to or inherited by anyone in the state.
Finally there is the more commonly known and more broadly incorporating 'assault weapon' by feature category.
These have defining characteristics that in combination make a weapon legal or illegal, features which may be removed or added and change legality.
This is what most people are referring to, and what the cop in the cited case determined improperly. Rifles for example are prohibited if they have a detachable magazine in combination with one of a list of other features. A fixed magazine is defined as one that requires a tool to remove, so if it cannot be removed with bare hands then it requires a tool.
The result is people can have all the other features they want if they make it so the magazine needs something besides a finger to remove.
These are features that are otherwise rare in California, and so immediately draw attention when seen.
A firearm enthusiasts or citizen that does there homework can put together such legal firearms, but as seen in this case may still be arrested and charged, and more importantly to them still pay all the legal costs associated with hiring an attorney. This means they are essentially fined thousands of dollars for breaking no law, because that is the cost to defend themselves and prevail.
As a side note:
The legal system is self serving, the more people disagree, don't get along, and readily sue each other over minor things the greater the demand and job security and higher the income for attorneys, prosecutors, judges, etc
They don't want everyone to get along, or to readily understand the laws, the more experts that must be involved in everything the more they benefit. If two people sue each other in court, or fight it out, the attorneys typically walk away the winners on both sides. Divorces are one of the prime examples, couples often lose their homes and other assets in paying for the attorneys to argue over the details. So they may have been together and accumulated those assets over many years, and the attorneys walk away with most of them.
This also means that if police or others do not have to pay the legal fees of those wrongly accused, there is no disincentive for the accuser to not continue such conduct, and there is financial incentive for the courts to keep dealing with the cases individually and not reduce their own source of livelihood.
That is not to say lawsuits are bad, they can be used to help clarify rights as in this case, but that the courts themselves have no desire to give rulings that reduce how readily charges are filed or people are sued.
As for this specific situation, it certainly has the potential to turn out worse in California. We just had politicians ban unloaded open carry of holstered firearms which was only legal more than 1,000 feet from all schools after all.
They could simply propose new legislation to outlaw the bullet button, or redefine a tool to mean something more complex or time consuming, as a result of such legal challenges.
