I was wrong, Mailspring uses the IMAP IDLE extension to wait for new mail delivery
The reason I was asking about the folders is that IMAP IDLE only works for one folder at a time. If you want real-time push for 10 folders (for example), it has to open 10 separate IMAP connections. Because of this, clients usually either only enable push for the inbox, or let you choose which folders to enable it for.