You're going to need to be more specific than just data dumping yet another news article covering the Trump claims.
Are you seriously saying the BBC agrees with Trump's claim the news media is underreporting terrorism? Or are you cherry picking some little nit and brushing it broadly over all of Trump's lies on the press coverage?
If you cannot be bothered to go to the link; there is little point in my explaining, in any case it would take too long if it was to be of value. A skeptic would go to the data (linked) and draw appropriate conclusions; my opinion is worthless.
However I am happy to say what the link is to. The BBC has gone through the list of 'under reported' terrorism events. They have then identified those they have and have not reported with links to those reports. In some cases they have been unable to identify any event, as linked above a least one event appears to be a murder by a mentally ill person, not a terrorist event. (Second link)
People can draw their own conclusions about the case that the US government is making.