Yes, the countries other than SA obviously do less testing - but they still do testing, they report positive cases etc. Namibia for example, previously detected over 1700 cases per day, but is currently on 8, they won't be testing like the UK, but if they could test that number before. Botswana, previously 800 currently 28, Mozambique 1800 to 8 again. Even Zimbabwe which is perhaps the least capable neighbour has is down to single daily figures from 140.
Sure there are some countries who manage essentially none, but SA's immediate neighbours aren't those, and for a porous land border spread hypothesis that is what you need. Maybe that does point to it arising in Gauteng, but then we need a patient who's incubated a variant for almost a year?