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At which point they will be stopped by Ubuntu login screen which requires user password (or fingerprint, pin, etc).

Basically there are two options for have to enter 1 password: 1. TPM disk encryption + login password. 2. Password for disk encryption and automatic login (doesn't support multiple user accounts well).

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no, I mean the attacker boots his own copy of Windows or Ubuntu, which is supposedly trusted by the UEFI keys. Will tpm somehow detect that it shouldn't unlock secrets for this boot?

Yes. The PCR state after booting won't match that of the regular system, so the TPM will refuse to give up the key.

This is contingent on the sealing policy including PCRs that would change as a result of booting a different operating system, like PCR 11, which, when booting a UKI, contains those measurements. Only sealing against PCR 7 would allow this attack, since the default platform secure boot policy would not need to change.

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