This is a myth!
Can you only absorb 30 G protein/meal for muscle protein synthesis? Probably not.
A study done by Tipton et al. (2016) shows otherwise:
2-group, randomized, double-blind, crossover design.
Group 1- 20 g whey protein isolate
Group 2- 40 g whey protein isolate
Total= 30 healthy, resistance trained (>= 2 sessions/week for past 6 months) males divided into Group 1 (n=15) and Group 2 (n=15)
Volunteers participated in two infusion trials (used to measure MPS response) after doing whole-body resistance exercise and ingesting whey protein isolate. Group 1 was given 20 g, and Group 2 was given 40 g whey protein isolate, immediately post-exercise.
Overall, Group 2 had greater MPS than Group 1, following whole-body resistance exercise.
Based on this study, consuming more than 30 g high-quality protein following heavy resistance training is more beneficial for muscle growth. The level of resistance training and the type of exercises you perform may affect the amount of protein you can absorb.