That is not a big deal or a red flag per se. Queen Maxima, who is a much younger and generally healthier person, had a urinary tract infection in China, which progressed to a kidney infection, and was flown back to the Netherlands and admitted into hospital.After the flight he still will go to hospital. That is probably a precaution after the long flight but it shows that he is not deemed sufficiently recuperated to further recover at home.
If King Harald has pneumonia, or any type of infection complication, it is totally expected that he would not be sent to recover at home, especially at his age.