This is a misrepresentation of the opposing viewpoint. No, the mere existence of a gender gap in an industry is not ipso facto evidence of sexism in that industry. After all, gender ratios in different industries vary widely, some industries have a higher ratio of women and others have a higher ratio of men. There mere existence of an imbalanced gender ratio in an industry, is not, in and of itself, problematic.
With that said, if you look at the tech industry, the constant stream of reports* of sexual harassment and sexist treatment from women that actually work in the industry is a pretty big clue that this industry, maybe, just maybe, has a problem with the way it treats women. And if you consider that female participation in the tech industry grew until the mid 80s where it peaked and has been going downhill ever since, it's difficult to conclude that the gender gap merely comes down to men and women having different interests.
So, no, the gender gap itself is not the problem. But it might be a symptom: if women feel like they are treated poorly in this industry, they are going to leave. Not because of lack of interest, or biological disposition, but because they do not feel welcome and are driven away due to the treatment they receive. And if that is causing the gender ratio in the industry to be lower than it would otherwise be, then that is problematic.
* Notable examples include but are not limited to:
https://www.nytimes.com/2017/06/30/technology/women-entrepre...
https://www.nytimes.com/2017/02/19/business/uber-sexual-hara...
https://techcrunch.com/2014/03/15/julie-ann-horvath-describe...
https://www.recode.net/2016/1/11/11588656/60-percent-of-seni...
http://observer.com/2017/06/justin-caldbeck-binary-capital-s...