Implementing search for E2EE mailbox is difficult problem (Protonmail is not doing the same). For search to be efficient, you would need to download the whole mailbox for your device. If you want to support as many users as possible, you can't. But maybe they could make it optional.
And they are very small team, offering mail for very low price (free for most), which has resulted on using Electron for producing the applications for as many platforms as possible. And it is a mess.
What comes to that Reddit post, it is two years old and they have supported mass selection for a long while for now.