Sure, but those two things would tend to work against it becoming a pandemic— unless it managed those two things but also kept its host healthy enough for long enough before becoming lethal to adequately spread it.
And what existing virus comes close to this trait?
But the Black Death mixed high contagion and high mortality as an actual example that shows they aren’t mutually exclusive.
Nobody said you claimed they were harmless. People are taking issue with your assertion that biological agents can be either contagious or lethal (not both), and therefore you discount its risk. This implied tradeoff between contagiousness and lethality simply is not enforced by anything in nature.
The natural emergence of a pathogen that's both highly contagious and highly lethal would be a much rarer event than the natural emergence of one that's either contagious or lethal, but we're talking about engineered pathogens. There is no reason to think that pathogens cannot be deliberately created that are both of those things.
I am clearly referring to this specific scenario. There is nothing in chemistry or biology or physics that prevents it.