You're correct, it can be done without the jig, if you have a pair of 20mm open wrenches and an assistant.
Of course it's easier with the jig.
Having the jig and even 1 wrench, it can be done by yourself.
You're lining the cam sprocket timing marks up to the top deck of the head - the jig just holds them there.
You can also make a jig yourself with 6" of angle iron, and a pair of 6mm or 1/4" bolts with nuts (pause the vid at 0:16 and you can see the dims)