Hi Paolo, thanks for reaching out, and apologies for the long response time.
Indeed, in async mode a Q3C functions such as q3c_dist() can currently not be used within ORDER BY. I you can live without it in an ORDER BY, the rest should work in async mode as well, e.g.:
ssql = '''SELECT * FROM decaps_dr1.object AS g WHERE q3c_join(13.92, -0.14, g.ra, g.dec, 0.01)'''
jobid = qc.query(sql=ssql, async_=True)
Note that the last argument to q3c_join() is the matching radius, i.e. it should not be zero as in your example. Also note that for your example coordinate ra/dec = 10/20 I don't seem to find any nearby matches, so I adjusted the example coordinates a bit in my example.