What is a pensioner?
These are elderly people who gets social benefits from the state or social funds. The pensioner's age is different in each country.
Is the "іо" supposed to be a diphthong, or are the vowels pronounced separately?
They are usually two vowels, but in a fast speech it can sound closer to "пенсьонер"
it is pronounced without a pause between i and o, if that's what you are interested in.
It is not a diphthong. Just two vowels, one after another.
Diphthongs in Ukranian do not work like that.