Zerodha was already India's largest broker more than a year before the pandemic happened[1]. The stuff mentioned in the post isn't something they're just starting to do now; they've been doing it since the beginning. They did benefit immensely from the pandemic stock frenzy, but they were very successful much before it happened.
I've been a user for more than three years, and have never seen a single pop-up or ad or basically any kind of nudge from Zerodha regarding signing up for any of their products. There are third-party add-ons (like Tickertape), but they explicitly make people aware that these are separate products with their own terms, conditions and fees if any. This is very much appreciated by users, and they tend to have fewer complaints per user in comparison to the competition[2].
They've grown primarily because they're good at what they do, and there is no catch regarding any of their products. Rest of the financial industry in India is a cesspool with a massive buyer-beware attitude, which is an instant turn-off for most people, but there weren't many other options until recently.
The one thing I find wrong about the post is that they aren't track-free; I notice Google Analytics being blocked on ublock origin. Hard to disagree with anything else.
[1]: https://economictimes.indiatimes.com/markets/stocks/news/ris...
[2]: https://www.chittorgarh.com/report/broker_complaints_exchang...