.. European Union leaders decided back in 2016, when the Brexit referendum was about to happen, not to interfere with the campaign since this was regarded as a British internal issue
I searched for terms like "eu leaders decide not to interfere in brexit campaign" and similar but could not find any direct official statements with regard to this.
I read the Wikipedia page about the 2016 United Kingdom European Union membership referendum but could not find anything related.
However, I also searched for direct interference in the form of speeches of EU leaders (Angela Merkel, Jean-Claude Juncker, François Hollande) before the referendum for example urging the UK to stay (or to go) and I could not find much. Search terms were "[name] wants UK to remain in the EU" or "[name] speaks about Brexit referendum" with a custom date range 01/01/15-06/23/16.
The most "influencing" statements may have been:
... "The United Kingdom will have to accept being regarded as a third country, which won’t be handled with kid gloves." ... Juncker May 2016
she hopes the UK will vote to remain "for the benefit of all of us"..."you can much better have an influence on the debate when you sit at the bargaining table and you can give input to those negotiations and the result will then invariably be better rather than being outside of the room." Merkel June 2016
"There will be consequences if the UK is to leave the EU, there will be consequences in many areas, in the single market, in the financial trade, in development, in the economic development between our two countries. ... It doesn't mean that everything will be destroyed, I don't want to give you catastrophic scenarios, but there will be consequences." Hollande May 2016
Indeed, it is cited that
Ms Merkel and the French President Francois Hollande have until now refrained from commenting on the poll for fear their remarks could be counterproductive. Source
The Wikipedia entry about the referendum does mention only a few European responses from opposition politicians or politicians from relatively small countries.
My conclusion: There likely was no official decision by EU leaders to not interfere in the UK-EU referendum, but for all practical purposes EU leaders acted as if there had been one at least with regard to official statements. The political analyst that you have cited probably refers himself to this.
Only in the last weeks before the referendum did EU leaders remark on the Brexit campaign and when they did it, they did it in quite a considerate, taken back way, mostly highlighting that there will be consequences and changes in case of a Brexit.