How do energy harvesting ICs like LTC3106 and LTC3105 safely charge lipos?
These 4.2V are unloaded, i.e. with battery disconnected.

The important bit with LiPos is that you control the charging current, and the limit is usually relative to the capacity of the battery.

The charging current is relative to the voltage difference between the charger output and the battery, and vice versa. High-powered chargers can provide more current than that, so they have to reduce their output voltage so the charging current stays in range. Trickle chargers cannot provide a lot of current, so their output voltage is the battery voltage plus the difference given by the available current.