"Strumming" -- or, put another way, playing chords rhythmically -- has been around as long as the piano (and probably longer, but I'll stick to the piano).
Using this kind of piano accompaniment has long been a mainstay of pop music. For example, the opening of The Beatle's "A Day in the Life" (1967) and Jerry Lee Lewis's "Great Balls of Fire" (1957).
The latter owes a debt to an earlier style, Boogie Woogie, in which repeated, rhythmic piano chords are a hallmark. Pianist Magazine credits the first Boogie Woogie recording to Jimmy Blythe's "Chicago Stomps" (1924) and the first hit song to "Pinetop" Smith's "Pinetop's Boogie Woogie" (1928).
But rhythmically repeated chords have figured prominently long before that.
Taking a leap about 100 years earlier, Beethoven was using "strummed" chords, for example in his Sonata in C Major, nicknamed the "Waldstein" Sonata (1804).
Repeated chords feature prominently in the opening of Mozart's Sonata in A Minor, K310 (1778). Haydn uses an accompaniment of repeated chords throughout the second movement of his Partita in Bb Major (Hob. XVI:2), which IMSLP tentatively dates to 1760.
The piano itself was invented in 1700, but did not gain prominence until the middle 1700s. During those 50-ish years, music itself was changing from "polyphony" to "homophony", the latter of which featured chords to a much greater extent.
However, that is not to say "strummed" chords didn't occur earlier. Bach, in the second movement of his Italian Concerto (1735) uses chords to accompany throughout. This piece, however, would most likely have been performed on harpsichord at the time, rather than piano.
Obviously there are vast differences in the way rhythmically repeated chords have been used. But as a technique overall, it's (at least) as old as the piano.
"Strumming" is, in fact, a piano technique in a different sense. It is an "extended technique" in which the strings of the piano are literally strummed, like a guitar or harp.
For some additional information, see Notation for strumming piano strings. It mentions a couple of pieces that include piano strumming.
And here's a video of pianist Annea Lockwood using some of these techniques.