There is at least one very recent precedent. On November 4th, the day after the 2020 US election when counting was still far from complete, the Slovenian Prime Minister Janez Janša congratulated Donald Trump on his 'victory'.
"It’s pretty clear that American people have elected Donald Trump and Mike Pence for four more years," Janša tweeted on Wednesday. "More delays and facts denying ... [the] bigger the final triumph for the President. Congratulations to the Republican Party for strong results across the US".
Source tweet here. I do not know whether the Center for Security Policy raised the same objections to Janša's congratulations to Trump as they later did to those leaders who congratulated Biden.
It's uncommon for world leaders to offer their congratulations before the loser has conceded, but I would not frame this as "unusual behaviour" on their part. Excepting Janša, whose statement obviously was very premature, they are acting more or less on the usual timing, offering their congratulations some hours after the outcome becomes clear. (Some exceptions - Putin, for instance, is holding out for official results.)
Trump, OTOH, has broken with the usual schedule - he has refused to concede, even at a stage where the losing candidate would normally long since have done so - and it is not at all clear that he will ever concede.
The framing offered by Shideler and some others in the Trump camp seems to argue that everything else should wait on Trump's concession. But nobody outside that camp needs Trump's say-so to tell them that Biden has won.