It's a command.
The linked answer appears to be conflating two different verbs: "blessed" (not a command), and "be fruitful" (command).
In verse 28, after "God said", there are five verbs in succession, in Hebrew they are all in imperative form:
- Be fruitful
- Rule/have dominion
God's blessing may well help them fulfil these commands, but God is still clearly speaking in the imperative.
Post-Script--discussion from the comments
In English we say things like "live long and prosper" and "have a nice day", but these are not commands. They are subjunctive statements which have an implied "may you" in front of them.
This is evident by comparing them to one of the most common subjunctive statements in English: Goodbye. Goodbye is a contraction of "God be with you" (much like the Spanish Adios). This is not a command to God, but a subjunctive statement: [may] God be with you.
In contemporary languages, this difference is more apparent in languages with a greater variety of verb conjugations. Let's use Spanish as an example. How do you say "have a good day" in Spanish?
It's Que tengas un buen dia NOT Ten un buen dia. The former is subjunctive and is grammatically correct; the latter is a command and is not how it would be said. In English we can miss this distinction because we have so few variations in our verb conjugations.
In summary, "live long and prosper" and "have a nice day" are not commands, and thus are not comparable to the imperative verbs in Genesis 1:28.
As noted by Jamin Grey in the comments, God can indeed give commands to wind, water, whales, fish, etc.
There was discussion in the comments about the Jussive conjugation, which could indeed be used in Hebrew to express a blessing or a wish--not identical but similar to the English & Spanish subjunctive wishes discussed above (see discussion here). This is true...but the 5 commands in Genesis 1:28 are Qal, not Jussive. Hebrew does have a way to express these ideas as a blessing (I bless you that you will have children)...but this is not the way the verse in question is written.
God is giving a command.