All plastics are polymers, not all polymers are plastics (DNA for example is a polymer).
As @Sewdhull mentioned some manufacturers started using "polymer" instead of plastic because of the negative associations people have with plastics in terms of durability. The plastic that Glock uses is a trade secret but most people seem to think it's either DuPont Zytel or something very similar which is just plastic reinforced with glass fibre.
Some airsoft manufacturers haver followed suit and started using glass reinforced plastics after ABS proved to be less than ideal for receivers (old TM M4's for example) and picked up on the use of "polymer" as a marketing differentiator because the vast unwashed masses neither understand the difference or care because they're led to believe that "polymer is better than plastic" when it's actually "glass fibre reinforced nylon is better than ABS".
As to @TheFull9's point, I very much doubt that "influencers" understand what they're saying as most of the time they're just repeating what the marketing blurb says, so at best they're perpetuating a lack of understanding and at worst misleading buyers. That said, the market will support itself at the lowest common denominator so is it really the manufactures/influencers who are at fault or the buying public for just not making themselves aware?