Labelling this a "referendum" or "pre-referendum" is nonsense and is highly likely to backfire.
The lines in the post mortem will be split without nuance: pro- and anti-Brexit.
As things stand, Labour - the only UK-wide party likely to make a significant showing - is still trying to straddle that line, no matter what lip service is paid to the prospect of its supporting a second referendum or what spin its spokes are putting on its conference vote or its manifesto, and no matter what its current MEPs - perhaps understandably pro-Remain - may think or say.
Pro-Brexiters no doubt will, as they did in the last general election, shuffle a Labour vote into the Leave column. Of course - they'll shamelessly grasp at anything to justify claiming support for their stance. If push comes to shove, they'll point to Tory and other Leave voters so disgusted with the EU and May's failure to deliver that they intend to boycott the election, and default to "the will of the people" and cite their 17.4 million as usual. If nominally pro-Brexit parties prevail, Garton-Ash will have scored an own goal.
Garton-Ash touches on the complexities of the D'Hondt system (Northern Ireland uses single transferable vote, which Garton-Ash doesn't mention). We have close experience of it here in Scotland, where we run a "modified" version of it for our Scottish Parliament election. It's very difficult to game through the sort of tactical voting he suggests.
Nevertheless, I do hope for a large turnout. MEP elections have long been the poor relation in UK electoral politics, not least because people (outside Northern Ireland, as said) find it hard to get their heads around voting for a party list rather than an individual, which is partly why UKIP gained a foothold in past Euro-elections and why most people would have difficulty naming their MEP. Partly I feel like that because it's not now a foregone conclusion that we'll actually leave (I'm not holding on to any false hopes, though), and if we do end up remaining in the EU, I want serious, committed MEPs to be in office, not spoilers or makeweights, especially as the political makeup of the next European Parliament doesn't look very promising at the moment.
If Garton-Ash wants a second referendum, let him carry on campaigning for it. This is a different beast.