No it isn't disbelief and any Muslim that said that needs to repent for their incorrect individual interpretation.
Allah says in the Quran that he forgives all sins except shirk. With regards to unjust suicide, which is due to a lack of patience and despair which are qualities of disbelief, then they will be punished in hell, and the Prophet (s) said they will be punished by suiciding themselves in hell over and over forever in the same manner they did in this life. The Islamic scholars interpret the word "forever" to not really mean for eternity, but for a temporary time, however long Allah wills, and if they believed, then they will eventually be taken out of the fire.
Secondly, there is a difference of opinion among the Islamic scholars with regards to suicide bombing. Some scholars say it is haram and quote what you mentioned, but also state that the ahadith where the sahaba went on suicide missions still had a small percentage of survival. Whereas other Islamic scholars state it is halal and rewarding to do that if it means killing the enemies and/or attacking a strategic area and they will not be punished, and they quote the aforementioned ahadith where sahaba went on suicide missions as evidence.
So when there is a difference of opinion among Islamic scholars, we Muslims are allowed to believe in any opinion, so long as it isn't an isolated opinion (otherwise it would be an innovation).