I mistakenly used 90% ethyl alchool to scrape some PLA off my bed; did I ruin it?

First of all, let's look at what kind of bed you have. According to a review, it is a "porous ceramic coated glass surface."

Your bed is fine...

Glass and almost all ceramics are virtually impervious to most liquids, be them alcohol or even most acids unless that acid is hydrofluoric acid. So on a chemical standpoint, your bed is most likely ok, just the alcohol managed to leech some of the coloration or deposit dust in the surface - which is no problem usually. The matte might even just be PLA stuck in the surface, so if this happens if you print in a different area, you know that is not damage per see it's a normal sign of use.

So from that standpoint, I see no problem.

...but there are safety issues with the bed design

Where I see a problem though is the construction of the bed itself: it runs on mains voltage and is heated directly, which can cause all kind of problems, especially breaking off the cable as there is no proper strain relief on the cable! Atop that, the review points out that the bed heats uneven, which can very quickly lead to stress and breaking of the bed.

Because of this construction, I strongly advise to refit a strain relief and use utmost care no to touch the bed during operation. Make sure not to hammer onto it and don't overheat it to prevent thermal cracking.