Orwell is wrong here, although not necessarily because he's being deceptive. There's any number of reasons we might expect his knowledge of desertion patterns to be incomplete. He served on a relatively minor, quiet front, so had limited first hand knowledge of the especially brutal battles, which the Republicans usually had the worst of and could cause mass desertion. His knowledge of the wider picture was limited by circumstances - while there was every chance of his being exposed to or even meeting deserters from the Nationalists, there was no likelihood he would ever come across a Republican who had voluntarily defected to the other side. Republican officials (much less newspapers etc) he might have gotten information from had every motive to publicise desertions to their ranks, and equal motive to downplay or deny the scale of desertions from their own side.

Perhaps the most fundamental reason Orwell might have thought this was true was because of when he served in Spain, ie early-mid 1937. This was the period of the conflict for which desertions were most evenly balanced - the initial coup had clearly failed, the Repubic won some crucial defensive victories around Madrid, enjoyed decent success in the air thanks to Soviet aircraft and there was some optimism that the Republic's economic advantages would work in their favour the longer the war went on. Even then, however, desertions were at best even - James Matthews estimates that for each side's 'Army of the Centre', the Republicans lost about 2,175 soldiers deserting to the enemy, while the Nationalists lost about 1,800. The floodgates really opened from early 1938 onwards, as Republican forces saw major setbacks on the Aragon front, and mounting losses meant having to conscript less and less suitable age groups, who in turn tended to suffer from higher desertion. By the later stages of the war, the line between 'captured' and 'deserted' grew increasingly blurry, as captured Republican soldiers deemed to be of little immediate threat were offered the chance of serving in the Nationalist army (generally in non-combat roles) in order to avoid future punishment, which many did. The Nationalists were also, broadly speaking, simply better at managing discipline among their conscripts, keeping a close eye on suspect individuals while doing their best to encourage enemy soldiers to desert via pamphlets and loudspeaker announcements.

Interestingly, foreign volunteers like Orwell were actually often among the most likely to desert in Spain. This wasn't usually a matter of politics, but rather reflected the strains of fighting in international units, which tended to be sent to the most active fronts and suffered disproportionate casualties - about a quarter were killed, and many more wounded. Some grew disillusioned with communist leadership (though not necessarily their ideals). Some simply had ended up in over their heads, or believed that as volunteers, they had the right to leave when they wanted. This meant that certain international units saw desertion rates of over 15%, although it's important to note that very few of these were to the enemy, and many were subsequently rehabilitated and rejoined their units. From my own observation, opportunity to escape without consequence played a role - for some nationalities like the Americans or British, they knew that if they could make it out of Spain (or in some cases even to an embassy), they would escape any legal repercussions. In contrast, the Germans - whose prospects at home were dim to say the least - saw much lower rates of desertion.

Sources

There is actually a full-length study of desertion in the civil war: Pedro Corral, Desertores: La Guerra Civil que nadie quiere contra (Barcelona, 2006). It's an interesting read, but far from unbiased (Corral is quite conservative) and I've found that his analysis doesn't really hold up whenever it has intersected with my own research. There's also a good chapter on desertion and similar phenomena in James Matthews, Reluctant Warriors: Republican Popular Army and Nationalist Army Conscripts in the Spanish Civil War, 1936–1939 (Oxford, 2012). For (English-speaking) foreign volunteers, see Robert Stradling, ‘English-speaking Units of the International Brigades: War, Politics and Discipline’, Journal of Contemporary History 45:4 (2010), pp. 744–66.