Could be, yes. But regardless of whether they came from or through or dipped in and out of the Steppe and/or Anatolia, what he suggests is that ATP3 had "a bit over half of Steppe ancestry", which is what the study claims to be testing. i.e. Similar to R1b Bell Beaker, which could also be descended from the same hypothesised pre-IE Anatolian R1b-M269 and might be not a Steppe population at all, but a brother to M269 Steppe populations.
Depends what he means by "Steppe" in this instance (to or from). If it was "an offshoot of cattle herders that directly migrated from Anatolia to Iberia during the Neolithic period," as he suggests, then no detour through the steppes would be required.
He doesn't say anything about a maritime route in the quote you posted (although that is possible). In fact, he proposed that they could have "spread metallurgy fairly quickly all the way to Iberia", indicating a trail (whether overland or coastal).
He hypothesizes that they might have "
directly migrated from Anatolia to Iberia during the Neolithic period," which implies by water as, at least, the more likely route. Or it could represent a later "migration of copper metallurgists from Anatolia to Iberia," once again bypassing the steppes. I take the latter to be his preferred theory, with a stopover in the Balkans to pick up the "Balkan-like EEF" they carried.
Maciamo explores both possibilities - descent from people who migrated "directly from Anatolia to Iberia" or people who "would cross the Caucasus and settle in the Pontic-Caspian Steppe".
The full quote, as opposed to your snippets, does
not support your interpretation:
"In other words it could be descended to the pre-Indo-European Anatolian R1b-M269, the group of cattle herders that would cross the Caucasus and settle in the Pontic-Caspian Steppe. So could it be an offshoot of cattle herders that directly migrated from Anatolia to Iberia during the Neolithic period."
That doesn't mean that M269 originating south of the Caucasus, and then taking a long detour over the Caucasus Mtns, and then across the steppes
and the whole of Europe, to arrive in Iberia, relatively unadmixed in c.3500 BCE is impossible. Just improbable.
In any case, regardless of what Maciamo hypothesises, it is clear there were at least two fairly similar sources of "Steppe ancestry" that became admixed into Iberians at different times, muddying the waters somewhat, especially as this "Steppe ancestry" was already admixed (to different degrees) with Balkan-like EEF before it arrived.
A few isolated samples over half a millennia might represent an input, but they don't look to be a migration or wave that transformed the population.