Learn Portuguese (Portugal): Acabo de ver o tráfego aqui no radar. - I have just seen the traffic here on the radar.

No, in European Portuguese, já combined with a past tense means "already" (for example, já vi means "I already saw it"). To express that you "just" did something a moment ago, you use the present tense of acabar de plus an infinitive, like acabo de ver in this sentence.

How would the meaning change if I applied this lesson's concept and said Já vejo o tráfego instead?

It completely flips the timeframe! As you are learning in this lesson, já combined with the present tense points to an immediate or assured future. Já vejo o tráfego means "I will look at the traffic right away" or "I'll check the traffic in a second", whereas acabo de ver means the action just happened.

I usually hear trânsito for traffic. Can I use it instead of tráfego here?

Yes, but they have slight nuances. Trânsito is the most common word for everyday street traffic and vehicles moving. Tráfego is a broader term for the volume or flow of things, and is the standard word used for air traffic (tráfego aéreo), web traffic, or things monitored on a screen like a radar.

Why is the verb acabar in the present tense (acabo) if the action happened in the past?