I can actually believe that it "can" be passed on by fleeting contact.
I had covid a few weeks ago. Very minor symptoms, but tested positive several times over about 10 days with the home testing kits.
Now the thing is, I am absolutely certain that I didn't spend more than fleeting contact with anyone over the couple of weeks beforehand. Our DS is away at Uni, so the only person in my household was OH who I spent any time in close contact with. OH is ECV due to cancer and continues to shield. I work in my own office, alone, and I don't have clients calling in. I walk to/from work (less than a mile from home), so I don't use public transport and rarely drive unless it's bad weather, so I'll have fleeting contact with other walkers to/from work, but I don't stop and talk/socialise with them. We do click n collect for our shopping once a week, so there's fleeting contact there, but we wear masks and gloves when handling the shopping, and the supermarket worker leaves the shopping on the trolley, so no prolonged contact. I also checked back on DH's hospital appointments for cancer, and the only one in the last few weeks was a blood test (others were phone consultations). We have no local family, so no visits to eachothers' houses. No going to pubs/restaurants, etc. We did go to a garden centre, but, we went mid week when it was quiet and again, any contacts were only fleeting as we passed other shoppers.
So, we really have to assume it was a fleeting contact that passed it onto me. We are 100% certain we didn't spend more than a few minutes (at most) close to anyone else, not friends, not family, due to OH shielding which also means I'm shielding.