Of course, a thief could try to bypass the login screen by e.g. booting with a different kernel command line, or a different initramfs. If you want to avoid this vulnerability, TPM unlock can be configured as a very fragile house of cards - the tiniest change and it falls down. The jargon for this is "binding to PCRs"
E.g. systemd measures basically everything that is part of the boot process (kernel, kernel cli, initrd, ...[1]) into different PCRs, so if any of those are different they result in differen PCR values and won't unlock the boot device (depending on which PCRs you decided to encrypt against). I forgot what excatly it measures, but I remember that some PCRs also get measured during the switch_root operation from initrd -> rootfs which can be used to make something only unlock in the initrd.