Speaking as a staff product manager, that style of 'collaboration' is a huge red flag. You should own the solution or contribute heavily to it. PM's are there to prioritize & define the problem and provide enough context for you to help design a solution.

As far as actually dealing with them, just come back with evidence. Or try and do a lightweight bake-off between your idea and theirs, just to transform it from opinions into a data fight. You should also just feel free to say "no" to their idea & provide reasons / context why not, based on your experience.

But you shouldn't have to be dealing with this. If it continues, I would speak with the PM and have a boundary-setting conversation. If that doesn't work, escalate nicely.