I noticed that when I removed the prediction=True and accessing the attribute prediction, like this,
with model:
pm.set_data({"X": x_test, "y": y_test})
predictions = pm.sample_posterior_predictive(trace=idata)
that the CSV has 75 columns of the form ('obs[k]', k) for k=0…74.
$ head experiment.csv
,chain,draw,"('obs[0]', 0)","('obs[10]', 10)","('obs[11]', 11)","('obs[12]', 12)","('obs[13]', 13)","('obs[14]', 14)","('obs[15]', 15)","('obs[16]', 16)","('obs[17]', 17)","('obs[18]', 18)","('obs[19]', 19)","('obs[1]', 1)","('obs[20]', 20)","('obs[21]', 21)","('obs[22]', 22)","('obs[23]', 23)","('obs[24]', 24)","('obs[25]', 25)","('obs[26]', 26)","('obs[27]', 27)","('obs[28]', 28)","('obs[29]', 29)","('obs[2]', 2)","('obs[30]', 30)","('obs[31]', 31)","('obs[32]', 32)","('obs[33]', 33)","('obs[34]', 34)","('obs[35]', 35)","('obs[36]', 36)","('obs[37]', 37)","('obs[38]', 38)","('obs[39]', 39)","('obs[3]', 3)","('obs[40]', 40)","('obs[41]', 41)","('obs[42]', 42)","('obs[43]', 43)","('obs[44]', 44)","('obs[45]', 45)","('obs[46]', 46)","('obs[47]', 47)","('obs[48]', 48)","('obs[49]', 49)","('obs[4]', 4)","('obs[50]', 50)","('obs[51]', 51)","('obs[52]', 52)","('obs[53]', 53)","('obs[54]', 54)","('obs[55]', 55)","('obs[56]', 56)","('obs[57]', 57)","('obs[58]', 58)","('obs[59]', 59)","('obs[5]', 5)","('obs[60]', 60)","('obs[61]', 61)","('obs[62]', 62)","('obs[63]', 63)","('obs[64]', 64)","('obs[65]', 65)","('obs[66]', 66)","('obs[67]', 67)","('obs[68]', 68)","('obs[69]', 69)","('obs[6]', 6)","('obs[70]', 70)","('obs[71]', 71)","('obs[72]', 72)","('obs[73]', 73)","('obs[74]', 74)","('obs[7]', 7)","('obs[8]', 8)","('obs[9]', 9)"
0,0,0,0,1,1,0,0,1,0,0,1,0,1,1,0,1,1,1,1,1,0,1,1,0,0,0,1,0,1,0,1,1,0,0,1,0,0,1,0,0,0,1,0,1,0,1,0,0,0,1,0,0,0,0,1,0,1,1,0,0,0,0,1,1,0,1,0,0,1,0,1,1,0,0,1,0,1
1,0,1,0,1,1,0,1,1,0,0,1,0,1,1,0,0,0,1,1,0,0,1,0,0,0,1,1,0,1,0,1,1,0,0,1,0,0,1,0,0,0,0,0,1,0,0,1,0,0,1,0,1,0,0,1,0,0,1,0,0,0,0,1,0,0,0,0,0,1,0,0,0,0,0,1,0,1
2,0,2,0,1,1,0,0,1,0,0,1,1,1,1,0,1,1,1,1,1,0,1,0,0,0,0,1,0,0,0,1,1,0,0,1,0,1,1,0,1,1,0,0,1,0,0,1,0,0,1,0,0,0,0,1,0,1,1,0,0,0,0,1,1,0,0,0,0,1,0,1,1,0,0,1,0,1
3,0,3,0,1,1,0,0,1,1,0,1,0,1,1,0,1,0,1,1,0,0,0,0,0,0,1,1,0,1,0,0,1,0,0,1,0,0,1,0,0,1,0,0,1,0,0,0,0,0,1,0,0,0,0,0,0,1,1,0,0,0,0,1,1,1,0,0,0,1,0,1,1,0,0,1,0,1
4,0,4,0,1,1,0,1,1,1,0,1,0,1,0,0,1,1,1,1,0,0,1,0,0,1,1,1,1,1,0,0,1,1,0,1,0,1,1,0,0,1,0,0,1,0,1,1,0,0,1,0,0,0,0,1,0,0,1,0,0,0,0,1,1,0,0,0,0,1,0,1,1,0,0,1,0,0
5,0,5,1,1,1,0,0,1,0,0,1,0,1,1,0,1,1,1,1,0,0,0,0,0,1,1,1,0,1,0,1,1,0,0,1,0,1,1,0,0,1,1,0,1,0,0,1,0,0,1,0,1,0,0,0,0,1,1,0,0,0,0,1,1,0,0,0,0,1,0,0,1,0,0,1,0,1
6,0,6,0,1,1,0,1,1,0,0,1,1,1,1,0,1,1,1,1,1,0,1,0,1,1,1,1,0,1,0,0,1,0,0,1,0,0,1,0,0,0,0,0,1,0,1,1,0,0,1,0,1,0,0,1,0,1,1,0,0,0,0,1,1,0,0,0,0,1,0,1,1,0,0,1,0,1
7,0,7,1,1,1,0,1,1,0,0,1,0,1,1,0,1,1,1,1,0,0,1,0,0,0,1,1,0,1,0,0,1,1,0,1,0,0,1,0,0,0,1,0,1,0,0,0,0,0,1,0,1,0,0,1,0,1,1,0,0,0,0,1,1,0,0,0,0,1,0,1,1,0,0,1,0,1
8,0,8,0,1,1,0,1,0,0,0,1,0,1,1,0,0,0,1,1,1,0,1,0,0,0,1,1,1,1,0,0,1,0,0,1,0,1,1,0,0,1,0,1,1,0,1,0,0,0,1,0,1,0,0,0,0,1,0,0,0,0,0,1,0,0,0,0,0,0,0,0,1,0,0,1,0,1
It just clicked for me that these correspond to the testing set size! So the obs_dim_2 is indexing the test set observation.