I think the parent was specifically referring to "I was playing", which in German would not be expressed using only forms of the verb. You would have to use an adverb, as in "ich spielte gerade". Or you could noun the verb: "ich war am Spielen".

Nit: isn’t “playing” itself “nouning the verb” via a gerund / participle construct?

I actually am not sure of the subtleties on this grammatical point.

"Ich war am Spielen" is not really Standard German. It's a West German/Ripuarian dialect form that people from that area sometimes use when conversing in Standard German. But yes, it does communicate the progressive aspect in the same manner that "I was playing" does in English. I agree that "nouning" the verb is an arbitrary distinction.

Even if you want to give this one to German, English also has the prospective aspect, which is missing in German (prospective "I'm going to play tennis", vs. perfective "I will play tennis").