isolinux (as the name says) is for ISO-images (CDROM) booting from BIOS (not UEFI).
syslinux is for booting from FAT, so it can (but not recomended) technically boot form ESP-partition (FAT32 type, it is a must for booting from UEFI_64, which ask for your BOOTx64.EFI)
summary to boot from UEFI firmware, with your files, you need:
- original tinycore.iso, real burnt on CD/DVD
- or USB installed TC bootable; so tinycore.iso -> extracted as kernel (vmlinuz) + initram (core.gz) AND boot-loader grub 2.x for UEFI installed into USB. so BOOTX64.EFI is basicaly "grub.efi". And of course isolunx.cfg is useless, grub configutation is grub.conf etc.
FYI: only syslinux 6.04 can boot from UEFI, if kernel and initram are on the same ESP patition as sylinux.EFI. (no chain-loading further other EFI progam; grub.efi can do chainloader other EFI like windows10, or it can boot TC from other ext2 partition on other disk)