Where there are humans, there is bound to be fraud. People are broken. They lie and cheat. It doesn't matter what country it is in the world or where the people are, where there is political power on the line, people will find a way to cheat. This is true in Hong Kong, especially since it is a wealthy country.
Voter fraud is widespread in the country. This is because people can fill out forms for other people without their permission when it is just done on paper. People can vote for others that live in their house without them knowing it. This might be done if people are particularly passionate about a candidate.
No, voter fraud is not a widespread problem in the country simply because the numbers did not add up in 5 polling places. There should be more studies into the country's election systems to determine if this accusation is correct. It is possible, but more research will have to be done.
I believe there are over 550 polling stations in Hong Kong, that means 5 have been found to have a problem, this represents less than 1% of the stations. More research is definitely needed to find out how many more polling stations, if any, have numbers that don't add up. At this stage there is too little data to say fraud is widespread in the country.