The real reason why he's designed the way he is was to mainly test medics and their choice of pocket buddies and focuses more on the pureness of critical hits. Yes, the large HP he gets is actually meant to confuse the player into thinking he is unbeatable. He becomes easier to defeat once you get a pretty good Medic to pocket a high damage player.
Originally, one of the updates was going to halve the HP formula from ((2000+n)*n)^1.04 to ((1000+n)*n)^1.04, but the pilot group who tested the newer HP formula had a field day defeating him rather quickly, not even 3 minutes into the round, mainly due to market garden "backstabs" (as he was designed to not be able to stun players or sentries on-rage, but rather, gain ubercharge for several seconds to give the boss a chance to fully reload his clip as he can only regenerate 20 rockets at a time, while his clip is 26 rockets).
It's similar to how my human sentry buster boss is set up - low HP, but 75% resistance to any ranged sources including sentry guns, and complete knockback immunity, but is designed to encourage players to use their melee weapons (which is why melee hits deal more damage against him than damage from ranged sources). Yes, his infinite ullapool caber is designed to give the impression of being overpowered and unbeatable as well, but once you have players who actually use their melee weapons, he's easier to defeat.