The background to this is much more complex than that article suggests. IGCSEs have traditionally been used by many independent schools for precisely the opposite reason i.e. because they were considered more rigorous and a better preparation for A-level. For a period (5-10 years ago) the government were encouraging state schools to take them too, or at least to have the freedom to do so e.g. here IGCSEs were also, I believe, used as part of the review for raising standards for GCSEs and the new 'tougher' regime.
Then about 5 years ago the narrative changed as the Gove reforms came in. IGCSEs were removed from league tables and progress measures so effectively forcing state schools to use the government's new system. Independent schools have more freedom to make choices as to which they use and tend to use a mix of GCSE and IGCSE depending on the best choice for each subject. I think I am right in saying that independent schools currently have about a 50/50 split between the two overall. Given that the reforms have been rather controversial (e.g. the English teachers I know hate the new GCSE syllabus and feel it has sucked the joy from the subject) it's not surprising that the IGCSEs have remained popular. Given the political football that GCSEs have been then you can see why there is an attraction in qualifications that are more shielded from Govt interference.
So the narrative of 'independent schools pick easy exams to game the system' is a bit simplistic. Also, I think I am right in saying that universities have always treated the two as equivalent despite IGCSEs being widely regarded as more rigorous for a long period.