Everyone using 18650's should know they're using them outside of what the manufacturer makes them for. Anything we do with them, even just charge them up, is not "officially" recommended. But we all do it. We're taking a risk. Putting a solder blob on them is another risk. Like I already said, keep a bucket of water nearby in case you screw up, and test the cells afterwards to make sure they're performing the same as before.
But this is different. They don't even think that soldering can be done safely by professionals. And the risk is not only when you're soldering. It could cause damage that could rear its ugly head at some future date. As the excerpts from the datasheet warn, it could damage the separator or venting mechanisms. The former could eventually lead to a dangerous internal short, and the latter could cause it to explode like a pipe bomb rather than safely vent as designed. You can't possibly know if such internal damage occurred because there is no easy way to detect it without very expensive equipment, e.g. X-ray / SEM (scanning electron microscope) imaging.