I think people are saying that a new PM in the middle of a parliament is not directly elected. When there is a general election the Parties have already chosen their Party leaders so people voting in the election know who will be prime minister if the party they vote for wins the election.
All that is correct. But it is also correct that we, the electorate, do not choose (that is, elect) our government's prime minister by a direct vote. Party members decide Party leaders, not the wider electorate. So the method is not different in the middle of a parliamentary term from the method during a general election. That is what is meant by saying it's not like a presidential election where the whole electorate vote for the country's president.
Army horses loose on London streets