If so, MTCR would've been breached by Western countries many times over in the past. It does not forbid any and every sale of missiles over 300km/500kg, but only transfers of such technology to countries that could not already build an equivalent system with domestic resources. Quite apart from the vagueness inherent in this definition*, Iran demonstrably has that ability in spades. Conversely, Iranian transfer of long-range missiles and loitering munitions to the Houthis is a pretty clear-cut case of a breach.
* It is a difficult determination and thus open to dispute. Can the Netherlands really build a Tomahawk equivalent on their own? The airframe and guidance, sure - a suitable engine would not be available off-the-shelf, though. It's well within Dutch industrial capability to develop at short notice, but how short a time qualifies and how do you judge that objectively and reliably? I mean, ultimately it's only a matter of time until Burkina Faso could achieve the same, though obviously a much longer period than in the case of the Netherlands. Where do you draw the line, and who gets to decide?
And that's before we get into the problem that the provisions of the regime focus exclusively on ballistic and jet-powered cruise missiles. Things like MALE/HALE UAVs or loitering munitions are not foreseen in the regulations at all. Do these constitute a cruise missile capability? They are unmanned and can deliver similar explosive payloads over comparable ranges, after all. This is a point Russia has actually raised, albeit in bad faith (but that doesn't mean it's not a veritable loophole...), to accuse the US of violating INF. So do, for example, Turkish sales of Akinci UAVs all over the place breach MCTR?
Last but not least, before you come up with the argument that Iran is only able to achieve the results it gets thanks to foreign dual-use tech, think again. If we apply that sort of strict standard, the number of eligible customer countries positively implodes and only exacerbates the difficulties with justifying certain sales practices that Western suppliers have engaged in. For instance, would Australia still be able to build long-range missiles (and thus be an eligible recipient of Tomahawks) if they were cut off from access to US, German and Japanese machine tools and electronics?