The question is a good example of form being a matter of interpretation. For me, though, Mozart K502 is sonata form.
Measure references are to the Breitkopf Mozarts Werke, Serie XVII edition, and time references are to the Anne-Sophie Mutter/André Previn/Daniel Müller-Schott YouTube video (below).
Note: The video originally linked in this post is no longer available. The video currently included features the same recording, subtract 12 seconds from the timings below. Post will be updated fully when possible.
Presentation of first key area
mm. 1 - 12 / 0:12 - 0:32
Establishes Bb Major, ending with a brief cadence in Bb major before immediately moving on. The primary motivic material is introduced in bars 1-2: the first motif in the piano, and the second motif in the violin.
mm. 13 - 40 / 0:32 - 1:17
Kicking off with the primary motif in Bb major, Mozart quickly begins a tour through a series of keys using both the thematic material as well as flashy scalar passages, eventually landing in F major.
Presentation of second key area
mm. 41 - 48 / 1:17 - 1:30
Mozart now re-establishes the primary motivic material in F major, the dominant key to Bb Major. But only stays just long enough to establish the new key area.
mm. 49 - 68 / 1:30 - 2:02
Again beginning the transition with the main motif, Mozart also again tours through several keys, this time introducing arpeggiated passages along with more scale-work. The whole thing ends with the same sort of trill that would signal the end of a cadenza, arriving, finally back in F major.
mm. 69 - 82 / 2:02 - 2:26
This serves to firmly establish F major by way of an extended cadential passage. It differs from the transitional passages in that it hews closely to F major.
Repetition of exposition
2:26 - 4:42
mm. 83 - 97 / 4:42 - 5:11
A songlike, gentle departure from the previous material. Apparently Mozart wants us to appreciate the graceful beauty of F major.
mm. 98 - 117 / 5:12 - 5:45
Again kicking off a transition with main motivic material, but this time the second motif, Mozart winds his way back, in true retransition fashion, to the dominant chord of Bb.
First key area re-presented
mm. 118 - 127 / 5:45 - 6:02`
A more-or-less repetition of the opening of the piece, except leading us toward Eb major.
Transition #1: variant
mm. 127 - 155 / 6:03 - 6:46
This time Mozart kicks off the transition to the "second" key area in Eb major. This is necessary since his destination this time is different. In order to keep this transition similar to the parallel one in the exposition, it needs to be transposed.
Second key area material re-presented in first key area
mm. 156 - 163 / 6:47 - 6:59
A parallel statement of the exposition's presentation of the second key area, but now in the home key of Bb major.
Transition #2: variant
mm. 164 - 184 / 6:59 - 7:32
A modified Transition #2 from the exposition, this time providing transition from Bb back again to Bb for the coda, as opposed to the early F to coda in F transition. Note that again the "cadenza-ending trill" appears to signal the beginning of the end.
mm. 185 - 198 / 7:32 - 7:58
A parallel passage to the coda of the exposition, firmly re-establishes Bb major for the final cadence.
Not a rondo
A rondo generally should have multiple repetitions of the primary theme in the primary key, with contrasting sections in between. In this piece, I don't find the primary theme substantial enough to stand on its own and does not have convincing enough contrasting sections. It also appears in too many different keys and contexts.