If FileVault was on, you’re already fine. If you don’t know and the machine is old, physical destruction is the simplest, zero-guess solution.
If the Mac doesn’t power on, the question isn’t “can someone undelete files,” it’s whether the data was ever encrypted in the first place. On Macs roughly from 2011 onward, if FileVault was enabled, the entire drive is encrypted at the hardware level, which means without your login password the data is essentially noise, no casual “yodal” or even typical recovery shop is pulling anything meaningful off it. If it’s older and didn’t have FileVault on, and it used a traditional spinning HDD, then yes, raw data can sometimes be recovered even after deletion, but that requires specialized forensic work, not something random recyclers are doing.
Since it won’t boot, your practical options are, remove the drive and either connect it to another Mac to securely erase it via Disk Utility (if it still works), or physically destroy the platters/chips. For SSDs, drilling through the NAND chips or fully shredding the board is enough; for HDDs, damaging the platters is what matters.