No method of encryption is secure against active attacks unless it is used together with an authentication method. This is equally true for ECB and for any other mode of operation of a block cipher and for any other method of encipherment.

Of course the use of ECB was inappropriate for this application, but despite this, the weakness of ECB is not enough to allow the decryption of encrypted SPICE models. Repeated blocks of 16 bytes aligned on 16 bytes boundaries would be very rare and even finding such repetitions in the short SPICE models is extremely unlikely to allow the guessing of even a small part of a model, which would still be useless, as only a complete model is useful.

So the use of ECB in this case is weird and it is a red flag about the competence of the implementer, but the use of ECB alone would be pretty much impossible to exploit for the decryption of SPICE models.