Aha, I've found the answer:
Asynchrony is a general term which is used to describe playing notes in a separated or not-quite-together fashion where they are written as if they should normally be played at the same time in the score, for example a chord to which an arpeggiation is applied, or a left-hand bass note and right-hand melody note both written on the same beat but actually played with one hand being placed slightly before the other.
It is apparent on many early recordings made of pianists who were born in the nineteenth century and has been the subject of detailed analysis in recent years (see Peres Da Costa’s ‘Off the Record’ cited in the bibliography). It is an area of performance practice that I find personally very interesting for its role in some of the most exquisite and in other instances most eccentric-seeming performances recorded by such artists. By incorporating it into my own playing I have found it of great effectiveness in realising the music of Chopin in particular.
Chopin as Heard: Asynchrony – An Introductory Case Study
So now here’s the resource that my impression of arpeggio rubato is more than only speculation:
Mark Arnest says in his paper
Why Couldn’t They Play With Their Hands Together?
Noncoordination Between and Within the Hands in 19th Century Piano Interpretation
The romantic pianism of a century ago differed from today’s more sober approach in nearly every respect - including attitude towards the text, tempo flexibility, agogic modifications, and even voicing. But no aspect jumps out at a listener more than the noncoordination of the hands: The older pianists don’t keep their hands consistently together.
This noncoordination was an almost universal feature of earlier pianism. A study of recordings and piano rolls by 118 pianists born between 1824 and 1880 shows that all but one engaged in the practice so some degree.
And he explains why they did it:
The purpose of noncoordination was to characterize the music, generally by heightening the expression and clarifying the rhythmic structure.
and referring to Malwine Brée, “The Groundwork of the Leschetizky Method,” Haskell House, Arnest quotes:
Neither should bass tone and melody-note always be taken precisely together, but the melody note may be struck an instant after the bass, which gives it more relief and a softer effect.
“More relief” suggests a musical accent; “a softer effect” could refer to either or both of two things: the acoustic phenomenon in which higher notes appear in the overtone sequence of lower ones, or the idea of rhythmic pulse.
and he names 5 reasons for this practice in the Romantic era like
and what they did:
Noncoordination as a Form of Tempo Rubato
for further reading: