What's a reasonable threshold for UVLO?
Yes, 1.55V is way too low for Li-Ion/LiPo. 2.6V is already on the very edge.

It has 1.55V UVLO because it works correctly down to 1.6V.

Not every single device in the world is designed to work with Li-Ion and only Li-Ion. If a converter says you could use a battery type that doesn't mean it will also suddenly do everything that the battery needs in ways of protection.

The main difference here is the presence of a PCB with many components vs just a chip.