Your premise that the pandemic would have "blown" up in December 2019 if there were some sporadic cases then is misguided. It takes time for the "domino" effect to kick in ... even with a high R value. Case in point being this late spring when the FLirT variant cropped up. It's much more infectious than the original Wuhan strain yet it took 3 months for cases to peak ... and that's in a society which isn't in lockdown and travelling without restriction.
I suppose my experience is still with my own Mother in Dec 2019 and my nephew many months later claiming she died of Covid.
She was in a respiratory ward with 20% lung capacity for at least three weeks but caught pneumonia.
She was allowed home and she died in my arms.
I'm sure you would agree with the following (maybe) -
My main argument is during that 3 weeks and her time at home there were at least 10 family and friends visiting her, cuddling her, kissing her etc.
With a virus has deadly as Covid surely at least one of us would have caught it?
At one point is was 24/7 care in her own home for some of us.
Three months later I'm getting 19 on the NHS Occupational Health questionnaire filled in by my boss, 9 was seriously at risk and he's ordering me home.
I just can't see how if my Mum had Covid in December 2019, how one of us didn't get it knowing how it was ravaging the public by late February/March.
And if my Mum had got Covid in a ward full of patients with bad respiratory problems, surely it would have raced through that ward with no Clinician knowing what was going on with extra RIPs than usual?