You're correct that the left isn't especially worried about procuring, supplying, and guaranteeing brides.
The presented solution to that problem from the right is "make women a captive audience through legal controls and normalizing trafficking. Increase resources by press-ganging the trans mascs and cisgender lesbians into heterosexual Womanhood. Involuntarily incarcerate and sterilize defectors."
What I think most incel heterosexual and cisgender men fail to understand is that there is a large population of gay and trans men in the same boat of being unable to find bonded relationship; along with the large proportions of cisgender and especially trans women who struggle to acquire affection without violence and ridicule for daring to want anything; the large numbers of immigrants, prisoners, and disabled people who are involuntarily sterilized for State interests, their capacity for continuation permanently stolen by force. All of us together receive violence instead of love.
The leftist solution is solidarity, which means learning from others facing similar problems and trying to do something reasonable and humane, together, about it. For example, one could work together with trans men to destigmatize things like, say, being a man who is short, and all the other ways in which trans men must navigate the intensely hostile terrain of masculinity. Bonding might be possible between the late bloomers who missed out on teenage dating... and those who were forced to navigate the field while wearing the opposite body to the one in which they wanted to be desired. Almost every nasty slur that can be weaponized to attack a cis man's body and capacities describes qualities we believe are intrinsic to and unchangeable for trans men.
But as far as I can tell (when last I checked) the incel community, which is different from simply the collection of all persons who are involuntarily celibate, is largely disinterested in dropping their privileges in order to work together in common cause; and claim that they are the lowest, most disadvantaged of people, and that no one anywhere is treated more cruelly.
Indeed, if we only asked for solidarity across the least icky line possible, I've been reliably informed that cis women cannot be incels. But the argument for this conflates sex and rape. Joining the left requires, at a minimum, a belief in female personhood to ride.
PS: I agree that calling random people incels as a slur is 1. ridiculous and 2. wrong. I'm writing above based on my experience with men I have met in real life who explicitly called themselves incels, were active in online incel communities, and who attempted in explicit terms to call me in as a surrogate mother and unpaid weekly therapist.
People are messy. The community needs to get its shit together, figure out less destructive modes of mutual aid and community care, and get productively organized. I recommend watching or reading Densha Otoko ("Train Man") to start, and following up with Fire Island.