They're not comparing things to each other, so they don't really have to be that consistent. "Less than five sachets of sugar" is a little more precise than saying "very lightweight", which is a useful level of precision as no one in the audience will know what a typical IR sensor should weigh, but greater precision is of no use to them since they will never handle one outside of a finished product, if at all. Also very low as well as very high weights are harder to relate to for most people, which is why elephants and sheets of paper are used so readily. If done well, this isn't bad style at all, though it is reliably off-putting to people used to read technical documents where precision levels carry a different kind of weight.

When it's put in the context of older generations weighing several killograms, surely stating the absolute amount would still bring enough context for the reader to understand the severity of change? Or what about referring to the weight both in grams and using a metaphor at the same time?

In a non-technical piece like this, it’s not the precision, it’s the consistency. If you measure things in sweetener packets, everything is in sweetener packets.