I don't agree. There's a lot wrong with the marketing copy of the website but I think that statement is quite clear.
For most countries where SIMs are commonly in use (i.e. not the US where CDMA is still a thing and not all consumers are comfortable with GSM), people generally understand that to change SIM you need an unlocked phone. I think the question is intended to answer whether you need a phone on a specific GSM or UMTS band, which you don't.
I was going to say the copy was glib going-on misleading. But then I read the about page[0], and my mind was changed.
<block>WhatSim is the stroke of genius of Manuel Zanella, founder and CEO of Zeromobile. In 2003, after his honeymoon in Kenya where he spent a fortune in phone calls, Manuel Zanella had the idea of creating a SIM card to reduce the cost of international roaming. In 2007 he created Zeromobile, the mobile company for low cost roaming that allows you to save up to 85% on the cost of incoming and outgoing calls, text messages and mobile Internet. And you can receive free in over 140 countries. Now with this new idea, WhatSim, he has created a revolutionary SIM card to chat for free with WhatsApp, the world's most popular messaging system.</block>
For most countries where SIMs are commonly in use (i.e. not the US where CDMA is still a thing and not all consumers are comfortable with GSM), people generally understand that to change SIM you need an unlocked phone. I think the question is intended to answer whether you need a phone on a specific GSM or UMTS band, which you don't.