Proper insulation is a prerequisite for heat pumps.
Where is this meme coming from?
These things (heatpumps, insulation) are totally orthogonal.
Quite the opposite, (e.g. ground source) heatpumps are specifically being recommended here in old, poorly insulated buildings (with maybe some historical or sentimental value); of course, because poorly insulated houses consume more heat, there is more (absolute) potential for savings.
For example, a well insulated house needing 9MWh/year of heat, heatpump with COP=3 saves 6MWh/year of energy. A poorly insulated house needing 27MWh/year of heat, heatpump with COP=3 saves 18MWh/year of energy.
Of course you can argue that with any COP/efficiency better than simply burning gas or fuel oil you'll need to buy less energy. But why stopping there if adding insulation lowers that dramatically more? So you can pay for a proper insulation or have to buy more/larger heat pumps to get the heat power needed. Without insulation you'll also need a higher flow temperature to keep your home warm (limit of typical heat pumps is about 40-45°C). The next point is that all the additional required electricity has to be generated. This will not happen over night and we don't have fusion power plants yet. It's about the whole strategy!
High distribution temperature is a problem of its own, it can be fixed by installing larger radiators or fan coil units, but if and when this is too costly, then just use air-to-air heatpumps which completely sidestep the issue.
Insulation is only an indirect factor here, because distribution temperature is basically defined by the ratio between insulation and radiator size. Thus, there is some synergy: improving insulation reduces distribution temperature even without increasing radiator size.
And as I said, you should be really doing both: fixing insulation and installing heatpumps. But neither is a prerequisite to other, and fixing these things work in any order.
Sometimes adding insulation is a very high cost operation, while a simple air-source heatpump can be ordered and installed in matter of hours and only 1500EUR spent. Or install two-three while at it.
But yes, longer term it makes no sense not to insulate new buildings properly, and also it makes no sense not to insulate old houses whenever doing some other renovations that enable easy addition of insulation.
And of course, always pick the low hanging fruits. In many buildings, attics are free, accessible space where ceiling insulation can be added in matter of hours.
Decision chart:
Get a heatpump:
Have hydronic heat distribution and want to keep using it for heating solely?
-> low distribution temperature (40-45degC during coldest times)?
-> yes: air-to-water heatpump
-> no: radiator upgrade plus air-to-water heatpump
-> no moneyz to do that?
-> see below
Don't have hydronic heat distribution at all.
Willing to retrofit it?
-> yes: retrofit large radiators / fan coil units / underfloow heating, plus air-to-water heatpump
-> no moneyz to do that?
-> see below
-> no: see below
"See below":
Install air-to-air heatpumps, one for each larger room, as many as you can easily afford. Existing heating system (maybe natural gas with hydronic distribution) remains in use, thermostats adjusted low enough so that radiators stay off in heatpump-heated rooms under normal conditions.