there's a reason 'safe' was in quotation marks, it's all relative to increased emissions.
The PPM does fluctuate based on natural carbon sinks, even right now, though the trend is an increase as emissions keep increasing. If we decarbonize efficiently, the atmospheric carbon will be drawn down, though at a slow rate. Faster if we have artificial carbon sinks like direct air carbon capture.
Saying that crossing 450ppm will be some kind of overnight apocalypse threshold sounds a tad sensationalist to me.