Pre-COVID19, students who applied and were admitted test optional to highly selective colleges generally had lower test scores than the overall average for that school. This makes sense as the primary reason why students did not submit test scores is that their scores were lower. However, there was also a small minority of students who did not submit scores in spite of score well above typical for the college. For example, Bates has a review comparing test submitters to non-submitters over a 25-year period at https://www.bates.edu/admission/files/2014/01/25th-Year-SAT-report-Stanford-6.3.11-wch.ppt . The slides mention that submitters averaged 80 points higher on math and 85 points higher on verbal. The slides also mention that both submitters and non-submitters averaged nearly identical college GPAs and graduations rates, in spite of the score differences. Several other test optional colleges have published reports showing something similar.
However, COVID-19 makes different. There are other key reasons why students may not submit besides just having a lower score such as COVID-19 making it awkward to take the SAT/ACT, students not seeing the point of taking the SAT/ACT when all the colleges that they are applying to are test optional, etc. A good portion of students with higher scores may not submit, as well as students with lower scores. As such, colleges will not necessarily assume that no score means low score. I believe some colleges really will not significantly penalize students for not submitting, as they claim. A few highly selective colleges have even gone test blind (not optional), such as Caltech, giving more certainty that not submitting will have little negative influence . Some test optional colleges will also apply notable changes in how scores influence admission decisions. It’s unclear exactly how scores will be used in admission decisions, but it is clear that it will be different from previous years.