I don't know whether that's true. According to
https://britishineurope.org/, no more than 20% of Brits in the EU are retired. Obviously, there are other things to consider and I'm far from an expert on the EU model on healthcare but with them numbers, it will be interesting to see if it's truly reciprocal.
'HOW MANY BRITS IN EUROPE ARE PENSIONERS?
Of the 1.24 million Brits in Europe, British in Europe estimate that around 200 – 250,000 are retired. So no more than 20%. The rest being of working age. There is a common misconception that the majority of Brits in Europe are retired. That’s simply not the case'.
Edit: just reading up a bit more on your point, there's about 90,000 EU retirees living in the UK so you're right there is a demographic skew but I'd still guess it's nowhere near enough to offset the extra 1.2 EU citizens in the UK.