In my grammar dictionary, it gives the example
but under the notes it goes on to say that 「そうだ」 can not be used with regards to past events or states.
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To answer your original question in the title (tl;dr), yes. The phrase is proper grammar.
Let's break this sentence up a bit:
That steak looked delicious.
What you're probably confused about was the そうだった part.
It's probably easier to look at the part after the particle by splitting it into
adjective + 〜そう and then
だった, which is the simple past form for だ. Therefore, the above sentence is really just a past form for this:
That steak looks delicious.
I think that the dictionary mentions the point of そうだ being inapplicable to describing the past is probably because you'd then have to conjugate そうだ into そうだった.
In addition, if you just look up
そうだ in the dictionary, it can also be used and understood as something along the lines of "that's right".