I am not sure I can agree with that either (unless your definition is "does it strictly adhere to end-to-end encryption standards", in which I'll agree with you that it's not the best).
Last I used Riot/Elements (the app the uses the Matrix network), I almost pulled my hair out. It was slow and buggy. Felt like I was using an alpha version of a software from the late 90s.
Telegram and WhatsApp are two very positive outliers in a sea of very bad messaging apps IMO.