A couple of potential answers:

* It's not about computers and them (Liz and Werner). It's about Cosmo.

* Computer dating is about algorithms and pattern matching. Cosmo didn't have any suspicion of Liz and Werner going on a date; even if he saw a type-mismatch, humans are complex and multi-faceted. But when he learned a computer program ostensibly made the match, his alarm bells went off.