If someone is unable to explain to the voters in a convicing way why they didn't want to take part in the primary, that's their problem. After all, if you're presenting yourself as a "pan democratic candidate" or something similar, it would make sense for you to take part in an open competition with similar possible candidates. You have the benefit of getting more voters on side if you win in the primary. If you don't take part and you can't explain it other than "I want to take part in the election to make myself feel good" or "I'm not as popular as X, so I'd lose", you're probably not going to win anyway.Anyone who didn't participate and refused to recognize the result would be ostracized and considered to be pro-Beijing camp which is automatically a death sentence for anyone in the opposition camp.
I can see the argument that a person might prefer there not being a primary because they'd have a chance of getting elected on a heavily split vote. But that's a personal preference. If some candidates agree not to stand so support can be pooled behind one person, that's their right. The point remains that the primary was legally optional and someone can stand even if they lost in it or didn't take part. The fact they may find it difficult to convince voters to vote for them is neither here nor there.
Really, the people most annoyed by this are the DAB and the HK government because they benefit from the pan-democrats having a split vote. The pro-CCP parties know that it's unlikely they'll ever get a majority of votes in the geographical constitutencies, so they feel threatened if their opponents can coordinate effectively.
There is no such thing as "block voting" in the geographical constituency elections. Everyone votes individually. Even if the primary results are useful for people who want to coordinate against the DAB, it's optional whether or not you follow it. The Opposition have no control over what happens in the voting booths. If a person wants to vote for candidate A and they're on the ballot, they can do so even if candidate B won the primary.In addition, voters would be signed up to form voting block before the election. Each voting block would be told to vote for which candidate. That is pure election manipulation and violate the election laws.