Yes, still Marketo track opens when the recipient’s email client retrieves the invisible pixel from Marketo's servers. IMO this is consitent with most other marketing automation platforms and ESPs. With the images being blocked from downloading, Marketo would never know whether a person opened the email unless they clicked a link that'd cause the email performance report to backfill for an open activity too, but that would not be reflected in a smart list as the email open activity won't be logged in the activity log until the client downloads the tracking pixel. FYR, this is one of the reasons why sometimes you see a difference in the number of emails opens in a smart list and an email performance report.
Also, all the above items along with the opens and clicks originating from the bots make these metrics a poor measure for gauging content performance.
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