I can neither confirm nor deny his competence, however I can see your point. It's frustrating to try to split while the 4 seem to twiddle their thumbs.
Kassidin, could've gone down perhaps because he saw someone roam and the waves were poorly positioned to just destroy the a tower/inhib. Now kass could've just done it to troll you, completely plausible.
I do think it's poor however to say that all players should abide to "If tryn is split, stay away." Especially when there are a plethora of reasons why they should help or even save tryn.
For instance... What if Allies are in a 1/4/0 and enemy are in a 1/2/2 (top/mid/bot) Tryn is at the 2nd tier with a massive wave and his allies are too, but the enemy Lux ults the wave, so they decide to swiftly rotate to tryn. By doing so they could possibly be granted the kill on whoever is top, 2 towers+ inhib or even the game if the two bot react too soon.
In your situation I can probably assume Kassadin did it with the intent to annoy you, but please do not encourage a niche situation to be the "go to" method.