I believe what it should have said is that 2s2p batteries are not as reliable as 2s1p, or in today's terms, 2s.

Not long ago the higher capacity cells were more expensive, so a 5000mah pack from a cheaper manufacturer could have been a 2s2p made with 2500mah cells. 2 in series for 7.4V, and 2 parallel for the total 5000mah with 4 cells actually in the pack. With parallel cells built into a pack where they cannot be balanced individually, there is more of a chance that if you have a weak cell it could cause problems.