А что насчёт глагола date?

Тоже можно

Uly, is "We didn't see each other for long." not ambiguous? At first look I read it like "Мы не встречались в течение долгого времени", т.е. долго не встречались.

Я бы сказал "We have been seeing each other not for long"
had been seeing?

It means both, depending on the context. You can mean "we didn't date 💕 for a long time" or "we didn't see 👀 one another for a long time"

However, "for long" would more naturally lend itself to an ongoing situation like dating someone, and "for a long time" to the absence of an action like seeing someone.

seeing or not seeing - both look like ongoing situation to me
So, "We didn't see each other for long." can mean that for a long time we didn't see each other. Hence the ambiguity that is best to be avoided IMO

You're not understanding me. "We didn't see each other for long" means that we saw each other but it (the relationship) didn't last a long time. It cannot mean that it has been a long time since we last saw each other because "not for long" doesn't have that interpretation. The only way to express the ABSENCE of an action for an extended period of time is through the phrase "for a long time" or "in a long time." So I repeat; If someone says "We didn't see each other FOR LONG" it can only mean that the relationship didn't last long -and not- we haven't laid eyes on each other in a long time.

Note that in both of my translations - consistent with what I just explained - I used FOR LONG, so there can be no ambiguity.

"We have been seeing each other not for long" is ungrammatical.

It seems I offended you. I didn't intend to. I was just explaining.