If you look at how outlook does this, you can sort of see what is meant by a true delete all not selecting page by page by page and then deleting... thats annoying.
Of course, this procedure is annoying and is not recommended.
Instead, you should use the 'search' option in AquaMail (the "magnifying glass" on the top bar) to filter all messages which are matching the search item.
You can find this option in message list view.
Enter your keyword(s), then swipe on the left from top to down (*) to select all displayed messages, tap on "Move to deleted" or 'Delete permanently" or "Move to folder... or [any other action].
(*) You have to enable the following two options in the settings prior:
1) Menu > Settings > Message list > enable "Pull to select" = Swipe down on the left to select
2) (in the same section) > choose "Select" All" means = "All loaded messages".