E-bike with downtube battery?
Even the lesser power Class1 bikes (mine = rear hub MahleX35 from 2020) sure make 20mph easy.

a non-drive side pic would reveal the BB area is not massive, which is the mid-motor giveaway

edit = 2nd the comment above and below about being 'physically fit and aerobically good' as possibly quite different than 'cycling fit' to roll XXmph for good distances (on a lightweight carbon fibre OR steel wonderbike).

Maybe I missed these questions - was his body/head position low, hands near the stem, pedal strokes extremely smooth, tires moderately narrow and pumped hard. Body position and efficient form matter, how much depends on the degree of differences.

Was he sneaky, predatory and snuck-up to blow-by 'effortlessly'? (us oldies do that on occasion)

snarky comment, = what did your power meter read?